27 years ago - Violet Harper is born.
11 years ago - 16-year-old Violet runs away from home.
8 years ago - 19-year-old Violet is beaten nearly to death by a drug dealer. She becomes the host of a dying interstellar energy creature, wiping out most of her memory. She falls in with the Outsiders, who name her Halo.
Violent Harper was clearly designed to give the Outsiders a young innocent character for them to protect. Everything about her character is built for that role, to the point where she isn't even given a complete backstory. She's a gestalt of a human teenage girl and an alien entitiy, creating a whole new being... perhaps the most deliberate attempt to create an ingenue character ever.
She IS a sweet character, though, and an excellent addition to the original team's lineup. Still, there really isn't anything else in the character's story; once her run in the Outsiders was over, she never really appeared again until the original lineup was reunited. Her status as an 'approximation' of a human makes it hard to find a role for her in the world after the end of the team.
Sometimes, this sort of story writes itself. The death of a character like Halo is something a group as close as the original Outsiders simply can't endure. It's a tactic often employed in DC, to have one of the teammates mind controlled into attacking the team and bringing about it's end, and one that fits here. It's a sad end for Halo, but it actually fits her story to tie her fate so closely to the fate of the team.