Tenjou Kaito
Yu-Gi-Oh! Zexal
Kaito starts out as a decent guy, but everything changes when his younger brother Haruto gets seriously ill. Dr. Faker convinces him that the Number holders and the Astral World are responsible for his brother's condition, which sends Kaito down a dark path. He becomes a Number Hunter obsessed with finding The Emperor's Key—believing it's the only way to reach Astral and save Haruto. The desperation basically turns him into a villain, even though his motivations come from a place of love.
Yu-Gi-Oh! Arc-V
This version of Kaito is a completely different person living in an alternate timeline. He was actually a successful professional duelist in Heartland City, in the Xyz Dimension, before Duel Academy invaded. When that happened, his family got sealed into cards, which destroyed him. He quit the Resistance to go solo, waging a one-man war against Duel Academy with the goal of sealing thousands of their soldiers into cards as payback. This made him dangerous enough that a bounty got put on his head.
His duels against Yuuya and Shun end up being turning points. They manage to get through to him, and he eventually joins up with the Lancers as an actual ally instead of just a lone vigilante.
Content compiled by AnimeList.moe from publicly available sources.
