White Diamond is the leading member of The Diamond Authority and as such, leader of Homeworld. As being the first Gem to exist, she made Homeworld what it is currently known. She is also the one responsible for Pink Diamond's shattering, and in a way, responsible for the Gem War. Her main goal as of passing the Gem War is to replace the Gem race with her Hybrid Gems. She's currently resides in the White Diamond Space Station. She serves as the primary antagonist for the season finale of both Season 4 and Season 6.
Physical Appearance
White Diamond is the largest Gem known to appear, taller than Yellow Diamond and Blue Diamond herself by an inch or two. Her hair is white and spiked upward to the back of her head. Her skin is silver, and her eyes are a very faint light blue with diamond-shaped irises. Her base outfit matches a standard Homeworld uniform, but also having a cape flowing behind her from her neck. Her own gemstone was a nice silver as well, right on her forehead.
Personality
White Diamond is a very calm, collective, level-headed leader. For many of her appearances, White Diamond's shown as an all-knowing being, and somehow always knowing what her subjects are up to, especially in regards to the Crystal Gems and their actions. She shows a kind of compassion towards her subjects, though this is rare to see, and even them shown to her more closer subjects and towards the other Diamonds. She holds little sympathy towards those who lost their lives, finding the idea of mourning a waste of time, and in the case of a shattered Gem, can easily be replaced.
White Diamond's main goals is progression and improvement to Gemkind, and sees many defective Gems as not even worth helping, as such spawning the overall idea that Off-Colors should be shattered as they're seen as not needed. She sees other Gems, and perhaps even the other Diamonds as less like thinking beings and more like assets and tools, and will discard a Gem type aside without a second thought if she can find a better alternative. This ideology prompted her to start work on Hybrid Gems, seeing that one can have multiple aspects of multiple Gems, for the price of one stone in a fraction of the time it would take for individuals to fuse for the same thing.
Her ideals also prompted her to shatter Pink Diamond, White seeing her as flawed and defective, and worked on reviving her in the way she wanted her to be, and this ideology also got her into attempting to permanently fuse Yellow Diamond and Blue Diamond to create "Green Diamond" as a better ruler for the Homeworld planet.
She does hold some trouble in wanting to keep her crime quiet until she revived Pink, as shown with her willingness to capture the Crystal Gems for their crimes during the Gem War, along with the Ruby Squad, Melanite, and Aquamarine in order to "preserve the purity Era 3 required". However, she's not afraid to admit to it, as she has complete control over Homeworld, and even threatens the Crystal Gems of another Gem War if it would ever go public.
She's especially infamous for pointing out others' flaws and loopholes in order for her to not be placed under any charge or crime, and making herself the correct one in an argument. This is shown when she still charges the Crystal Gems with their Gem War actions, despite saving Homeworld of the Osicone, and uses them as the culprits that killed Pink Diamond just so Blue Diamond and Yellow Diamond will see things her way and not argue with her about it. In spite of this, she will not admit that she is wrong, and believes that all of these actions are to benefit Gemkind as a whole, even though it mostly benefits herself.
History
She was the very first Gem to come to existence, and the creator to the other Diamonds. As the leader of The Diamond Authority, it was her who created the rules and decided what Gems are assigned to what task. During her early days, she began work on her White Diamond Space Station.
During Pink Diamond's reign on Earth, White Diamond at first didn't bother too much about Pink herself. However, when she tried to convince them that this was her colony, White saw her completely flawed and stated the colony will be completed "with, or without her". She confronted her sometime after this with plans to shatter her, just to run into "Rose Quartz" just before doing so. Believing she killed her, and in order to keep her name clear, she altered the memories of her members, including Eyeball, along with other witnesses such as Lapis Lazuli, in order to place the blame onto Rose Quartz. It was her that began the foundations for the Gem War.
She heard word of Garnet's fusion, and this gave her inspiration in order to study on the genetics of Gem fusion, finding that she could make Gems that can serve multiple purposes for the price of one Gemstone. So as the Gem War went on, she went to work on her Gem Hybrids, creating Pearlis, Flint, and other types, all the while trying to revive Pink Diamond the way she envisioned, and soon giving the green light for the Cluster Gem project, her planning to use it to remove all evidence of Pink Diamond and possible evidence of her involvement.
Season 4
One day, she called down Onyx to planet Earth in order to capture the Crystal Gems. once she brought Sapphire back, White stopped by Homeworld to check in on Yellow Diamond and Blue Diamond, only to find they had more respect to the Crystal Gems now, especially after finding the Mega-Fusion monument of the Osicone's defeat. She brought both Diamonds into her Space Station, and after a brief tour tested their loyalty by instructing Blue Diamond to shatter Sapphire. When it didn't work, she tried the same idea on Blue's Pearl instead, and when that didn't work, she poofed her. She instructed Yellow to do so, but Yellow saying she wanted to find what other punishment could be done first. White showed her the science lab (the exception being chamber 77). After doing so, she was confronted by Yellow and told what had happened, but she claimed that didn't matter, as their involvement in the Gem War still stood.
Eventually she found the Crystal Gems and got half of them captured herself, and brought everyone to her throne room. When she found out that the Diamonds never knew of her death thanks to a false report by Lapis, she revealed that Pink Diamond had been shattered, putting the blame on the Crystal Gems in order for the Diamonds to join her side, only to find it didn't work. A fight eventually broke out, White diamond taking involvement for a bit, but when it began to escalate, she called the whole thing to a halt, and offered to let them go if they could defeat her in a duel. She was confronted by the newly formed Earthstone, and the duel commenced, White Diamond quickly getting the upper hand. She ended up almost shattering Lapis Lazuli during the fight, which made Lapis go berserk, and actually poofed her. When she reformed, she had everyone else leave, giving Yellow Diamond information about The Cluster.
Season 5
During the season, White Diamond checked on Blue, finding she was still mourning over Pink Diamond. She tried to make her see reason, and when it didn't work, she straight up forbade Blue from visiting Earth. While she said it was to get her mind off it, it really was to make sure she didn't find anything in relation to the incident.
When Grootslang stole Blue Diamond later on, she called on The Trial against the Crystal Gems for her shattering. During the trial, she summoned Steven himself and used her abilities to make him describe in his own words how he shattered Pink diamond. White went into a breakdown as an act to better place the blame on them. In the end, White Diamond sentenced the Crystal Gems to banishment of Homeworld, and any and all Crystal Gems would be harvested if they return.
She held another meeting later with the Mane Six when she found out Acid and Cyanide had involvement with Blue Diamond's disappearance, but oddly allowed them to go after she was left with them.
Season 6
She went down to Earth herself at one point, finding out they were dangerously close to figuring out the case, and had them all imprisoned for defying her previous trial sentence. She only left the Rutile Twins behind, as she saw them too damaged to be of use to her Hybrid project. she instructed her Gems to remove any and all evidence from the Crystal Temple, and she brain-wiped everyone in Beach City to be sure they didn't involve in her plans.
When she was finally confronted by the Diamonds in regards to the Pink Diamond case, she finally came clean and told them all the exact details to her ultimate goal (including reviving Pink Diamond), though mainly because both Diamonds were next to be hybridized to form Green Diamond. Before sending them off, she pointed out Peridot amongst the crowd, and gave her one more chance to "redeem" herself, giving her the simple order to release The Cluster, being called a clod for it. She then got Pearl to send them off to the science lab. She stopped by there later on, and continued talking, filling in all the remaining gaps in the Pink Diamond case for them. She made it clear that they had nothing left, and that she had brought nightmares to them. When accused of being a monster, she brushed it off saying she's no different from Humans down on Earth, who she believed shared the same hatred as she did. She got them silenced by threatening to trigger another Gem War with them if they continue, and telling them as well that they literally have nothing left to return home to. She intervened during the fight, and attacked Connie and Lars: crushing Connie's legs, and killing Lars as well. She got into another talk with them, but realized too late that it was a distraction, and she found her station without power. She had her pearls sent off to both turn on the backup power, and to see where they were going.
Season 7
Not long afterwards, she was forced to leave her own space station, and during her flight she made a broadcast to the rest of Gem kind. She admitted to shattering Pink Diamond, as she was following the rules she had made and pointed out that her being a diamond made no difference. she also added in that the Crystal Gems, humans, Off-Colors, and Mane Six were now wanted criminals. She also noted that they will begin to see more hybrid gems, but not to replace all Gem kind as previously stated. She made a secondary broadcast specifically for the Crystal Gems, telling them of Pearl, Aquamarine, and the Zoomans being left in the station. She told them that she didn't start up a second Gem War, but reassured them that she will break them.
It is revealed in Diamond Days that her defeat on the White Diamond Space Station convinced her that she had to "be better". It's not specified how long this process took, but it eventually lead her into a almost maddening state, and to forcefully hurt herself in means of a specialized Gem Destabilizer, forcefully altering her form in hopes of becoming better.
She doesn't physically appear this season until the very end, meeting up with Flint in isolation (and having a different form). Thanks to a previous encounter agreement, Flint gave her Starswirl's Journal, as she gave Flint a Gem Book in a trade. Somehow, she already knew about Pink being Rose, finding it "ingenious".
Season 8
White Diamond's form is drastically altered after her Gem Destabilizer incident.
Eventually, she returned to Homeworld, and forced a full takeover of it by threatening to destroy the planet completely, tricking Yellow Diamond, Blue Diamond, and Pink Diamond into thinking she's holding the planet together when really she just used a spell to keep the planet from drifting apart, only doing this so they wouldn't immediately kick her out on her arrival. Her takeover became full by the time the Crystal Gems arrived onto Homeworld, with White Diamond being the first to greet them. During Steven Universe's first arrival with her since her last defeat, she treats him as if he were the true Pink Diamond, not letting him speak a word and in the end sending him off to "his" room. She later contacts the Diamond Authority, informing them that she will be visiting them in due time, again not letting them speak a word in. When she arrived to the main Diamond Palace, after getting reacquainted, she offered the idea of a "getaway" with all of the Diamond Authority, explaining how it would be something Pink Diamond would've wanted. This is later revealed to be a demand rather than a request, making them all go rather than giving them the decision.
While on this vacation, White diamond became the main subject of Steven's friendship lessons, him believing she doesn't truly understand friendship and him wanting to do so. She treats these lessons more like one of Pink Diamond's games however, not taking them too seriously, but she did use one of the lessons while on Heiligdom as an excuse to go after Yellow Diamond and attack the K-Bora, destroying their home in the process. While on Farid, she kept her involvement at a minimum but did tell Steven about her true feelings of the lessons.
On their last visit, she sets her plan to action, deliberately stalling directions so they could pass by Orthix, a planet they wanted to avoid. She refused to listen to Sapphire's warnings, and only moved the ship so it could be struck by a passing asteroid, making them crash-land onto Orthix anyway. She waited until Steven and Connie Maheswaran had left the ship to find Lapis Lazuli, Pearl, Bismuth, and Peridot to "make preparations" - controlling the minds of everyone on board to make them "perfect". She did the same to the rest (excluding Connie and Steven) when they got back, and revealed she was faking her friendly behavior the whole time to finish them all off. She explains to Steven how this method was, technically, the same thing as his lessons but only "faster", and pointed out this friendship as just his way of trying to control her - something she already can do. She later explains to Steven how, in essence, he is Pink Diamond and insisted she will make him absolutely perfect, and she almost succeeds when she suddenly starts to hallucinate thanks to Orthix's effects, giving her a nightmarish vision of self-doubt to the point of collapsing in defeat, releasing everyone from her mind control and her true status as an Off-Colored Gem. She ultimately decides to take Steven's words to heart, and is accepted back by Blue Diamond.
My Little Universe II
During the final season, she and Discord were caught and brought before the Spirit Council for her god complex and powers over her own world. As divine punishment, she was drained of all her powers by The Spirit, and left back on Earth in her weakened state to "expose her to her peers". She kept herself isolated at first throughout much of the season, afraid of what the rest of Homeworld would think of her if they ever saw her like this. Overtime however, thanks in part to the other Diamonds, she eventually overcame her timidity and showed herself before the royal court once more, living back on Homeworld again.
She doesn't regain her powers until the finale. She and the other Diamonds took refuge in the Earth Moon Base while Sumu was invading their planets, evacuating Empire City beforehand.
Abilities
Mind Alteration
White Diamond uses a form of mind control over some of the Gems, usually as a means to keep them quiet or see things her way, though this ability is barely used or not really required all that much. She notably used this to change the memories of those witness to Pink Diamond's shattering so they will remember Rose Quartz shattering her, though this isn't fool-proof, as Eyeball recalls Rose Quartz as being "taller" than usual. A similar form of Mind control is used on Pearl, making her body fight her and silence her in order to keep the event quiet if she ever tried to say anything about it.
Though unconfirmed, she might've used this during the Trial, making Steven recite the events of Pink Diamond's "shattering" in order to have the crimes further placed on them, Steven claiming he just got scared.
Genetics
After beginning her study after the discovery of Garnet, she has grown very knowledgeable in the study of Genetics, namely in the study of Fusion and hybridization. Many of her projects can be seen within her appearances with her Hybrid Gems and experiments. She also has extensive knowledge on the process of Gem creation, and knowing how different Gem types work and operate.
Gem-Tech interfacing/Engineering
White Diamond had built the Space Station into the most advanced station of Gemkind, many of the technology on the ship up-to-date and highly advanced when compared to Homeworld itself.