Tenjou Kaito is a Number Hunter. He was originally a nice person, but when his brother Tenjou Haruto fell sick, Dr. Faker told him that it was due to the Number holders and the Astral World, thus Kaito turned evil trying to seek out the holder of The Emperor's Key which would lead him to Astral.
Kaito in Arc-V is an alternative version of his Zexal counterpart. He was a famous professional duelist in the Xyz Dimension's Heartland City before the invasion of Duel Academy, after which he becomes a member of the Resistance. After his family was sealed into cards by Duel Academy, Kaito abandoned the Resistance and began a one-man vendetta against Duel Academy, vowing to seal thousands of its soldiers into cards and earning a bounty on his head as a result. After his duels against Yuuya Sakaki and Shun, Kaito has a change of heart and he becomes an ally of the Lancers.