Out of all the friends and allies Steven made along the way, no one is as dour and cynical - barring a Lars or two - like Lapis Lazuli is. At her highest point, she's apathetic. The rest of the time, she's negative, doesn't suffer fools greatly, and carries a ton of baggage. Lapis is a resentful Gem with lifelong grudges and her sullen personality means she's not the easiest person to get along with. She's so pessimistic that she often creates problems around her instead of maintaining enough positivity to find a solution. Even at her most relaxed, Lapis tends to keep to herself and rarely relies on others. All she wants is relief from the constant sea of nonsense thrown in her direction.
It's hard to blame her though, Lapis' life seem destined to meet with disaster. Thousands of years ago, when Homeworld was in the middle of colonizing Earth, Lapis was one of the many Gems visiting the planet. She was only meant to stay for a short time, but the war between Homeworld and the Rebels caught up to her. She was attacked and reverted to her dormant Gem state. Mistaken for a Crystal Gem, Homeworld placed her Gem on a mirror and bombarded her with questions she could not answer. Homeworld eventually fled Earth in a hurry, leaving her abandoned at the Galaxy Warp Station. It would be sometime before Pearl found the mirror, unaware of who dwelled inside.
By the time Lapis escaped, she had built up a dump truck's worth of anger. She gathered all of Earth's ocean to create a pillar in a vain attempt to reach Homeworld, but to no avail. Steven's healing ability restored Lapis to her full power, giving her the means to depart. She later returned unwillingly with Jasper and Peridot in tow. She had been trying to warn and protect Steven from Homeworld's arrival, but it was too late, Jasper nearly overpowered the surviving Crystal Gems. Lapis, sick of being everyone's rock puppet, fused with Jasper, transforming into Malachite. While Jasper assumed Lapis joined her side to defeat the Crystal Gems, Lapis had a ulterior motive: she dragged Jasper to the bottom of the ocean with her in order to save Steven and have some degree of control over her life.
The Crystal Gems eventually saved Lapis and she's since made a home for herself in Steven's family barn. Lapis is the victim of horrible trauma, but her resentment meant she's pulled harmful acts to other people as well. If there is any hope for Lapis' struggles, it once again falls to Steven. She may have issues to resolve, but Lapis will throw down anything to support him. After all, the boy has shown her nothing but kindness. Perhaps his warm heart will help Lapis conquer her own demons someday.