Eyeball is a name given to one of the rubies amongst the Ruby Squad. She in particular is identifiable for her Gem on her eye, and for being a veteran from the Gem War. She was sent down with the other members of her squad to investigate the Tunguska Gem Facility.
She holds the same body type as any other ruby, her gemstone located on her eye, and wearing a standard Yellow Diamond associate uniform.
Eyeball is a Gem more on action than actual conversation, often staying quiet as the team worked things out, or acting upon things herself. She is the most observant of the group despite only having one eye, able to recognize Ruby out of their group when the others didn't.
Not too much is known of her backstory, though it is implied that she was involved in the Gem War in some way. She did lay witness to Pink Diamond's shattering, but had her mind altered to think Rose Quartz shattered her, yet she still thought Rose was "a bit bigger than usual".
She makes her debut when her team arrived on Earth to locate the Tunguska Gem Facility. Eyeball remained quiet throughout the whole endeavour, aiding in getting the security cameras working. she only spoke at the very end of the mission, once collecting the Nuckelavee Monster, stating that Ruby was indeed a Crystal Gem, and she knew the whole time. She only didn't say anything because she was help to them finishing their work.
She doesn't appear again until later on, and was located by Jade during her own investigation in the Pink Diamond case. She was individually brought out by Jade and asked about Earth, to which Eyeball explained how Earth's production started out well, only to be stopped when Rose rebeled. She noted how Rose looked bigger than usual, and appeared "different", implying that White Diamond's memory-altering didn't entirely work on her, before she had to leave for the rest of her squad. She and the rest of her squad ended up captured sometime later, and rescued off of the White Diamond Space Station.