Lion is a magical pink lion befriended by Steven that occasionally assists the Crystal Gems. Lion's mane contains a pocket dimension that houses many of Rose's belongings (and, formerly, Bismuth's bubbled gemstone).
Physical Appearance
Lion has round ears, pink fur, black eyes, a soft light pink star-shaped mane, and a dark brownish-magenta heart-shaped nose. When Lion uses his magical abilities, his eyes and mane emit a white glow. There is a tuft of soft pink fur at the end of his thin tail.
Personality
Lion is an enigmatic creature whose sentience and sapience is somewhat questionable. Much like a normal animal, he rarely changes his expression and for the most part, he is quiet and docile. Though he is considered a "pet" for Steven, Lion looks after himself and often sees to his own activities. While Lion does not seem to acknowledge Steven most of the time, he does appear to understand Steven to a certain extent, responding whenever Steven needs him in mysterious ways. He does show some bit of understanding to others as well, shown when Sadie Miller convinced him to take them to Homeworld after the Gems had been taken by White Diamond.
However, in "The Results Are In", Lion shoots Steven a disapproving glare before leaving with her, showing that he shares Connie Maheswaran's anger at Steven over his surrender, as well as him treating the situation as nothing, and ignoring her feelings about it. Lion stays with Connie afterwards, presumably to give her comfort and companionship during her and Steven's estrangement, as well as teach Steven a hard lesson over his actions; Connie later rides him over to Steven's house to try and work things out in person after failing to do so over the phone, only to discover that he had left. It's unclear if he had forgiven Steven as of yet.
History
Once a normal lion (presumably owned by Rose Quartz in the Desert hundreds of years ago), Lion eventually died through unknown means. Rose Quartz was able to resurrect Lion through her healing powers, giving Lion pink skin and magical abilities. After being entrusted with many of Rose's belongings, Lion was left to roam the world. He presumably faced a Silver Lion in the past, and after a failed fight, roamed in the Desert before being found by Steven Universe.
Most of his appearances mainly place him as a side character apart from a few notable moments:
Season 1
After finding Pinkie Pie after Steven's party, he grabbed her and took her back into Ponyville through one of his portals, deliberately bringing her to Sugarcube Corner to remind her how much she had done when compared to Cheese Sandwich.
When he initially discovered Steven Universe (corrupted at the time), Jade, and Peridot, he made sure to inform Peridot through visuals of Sombra's involvement with the Gems. He went to Beach City, and after running into Sombra, faced against Tirek. It was him who took him down by cracking his gained Gemstone.
Season 3
When the group was trapped in eachothers' bodies due to Discord's Body Puzzler, Lion discovered their predicament and, despite Peridot's pleading, he went back to Ponyville and in his own way informed Rarity about what the problem was, using charades and visual clues to do so.
Season 4
As Steven, Connie, Amethyst, and Spike went off to the rock show in Ocean Town, he and Blue decided to go off after him, him helping Blue get away from Greg Universe as he was sleeping, and Blue was tied to the van. He and Blue went to a bus stop, which Lion actually tried to use by taking money from the Crab Shack, but of course it didn't work as he was a lion. Lion followed Blue out of town until they did eventually relocate them at the rock show.
Season 5
After having a rough dream about Silver Lion, he went back to his home land in order to fight against the lion once again. His first fight ended very badly, him injured severely and he tried to escape through a portal, but got himself beaten up and thrown into it. He woke up at Fluttershy's cottage, but despite his injuries, he ended up leaving and got Angel to move out of his way. He created a portal and got himself back to his home land again for a rematch with Silver Lion, getting brutally beaten once again and thrown back through his own portal. The third fight occurred at the Barn House, which Lion finally becoming the winner and allowing Silver Lion to go home on his own paws.
When Beach City became evacuated, he and Blue were the only ones remaining, and they first discovered Acid and Cyanide's true intentions towards the city. He managed to relocate the evacuating citizens, and managed through pictures just enough to tell them about the Beach City Flooding.
My Little Universe II
He briefly appears alongside Blue and Spinel in the three-part finale, going to the Supreme Kindergarten to try and find the remaining Harmony Gems.
Abilities
Dimensionkinisis
Lion has shown to be able to manipulate dimensions with relative ease, gained possibly after being revived. Much of these abilities allow him to create portal to different dimensions and worlds, also able to create multiple portal through a single stream though this has shown to weaken him.
He also possesses a pocket dimension inside his mane, though while he himself can't access it, his owner Rose Quartz, and then later Steven Universe, are able to store objects within it.