Zero and Kaito have been friends for a long time. Their history is dated back to the days of Zero’s childhood, in which he was a trainee for Vampire Hunting, training alongside his twin and also training in the shadow of number one ranked Vampire Hunter, Toga Yagari, who took young Kaito in after his sensei had been critically injured, and was seen to be not healthy enough to carry on teaching his vampire hunting methods to a very young Kaito. With that in mind, the long bond between Kaito and Zero begun, and as time went on it seemed to only get stronger.
We Forever Stay Perfect EnemiesEdit
Upon the slaughter of Zero’s family it seemed as though the two (Zero and Kaito) were separated after Yagari-sensei left Zero in the care of Kaien Cross to go and investigate the murders of Zero’s parents and twin brother (who was later found to be alive and well, allied with Shizuka-Hio). However, when Yagari returns and a lot of crazy, un-tamed problems are faced he then insists Kaien Cross to recruit Kaito as not only a part of the Hunters Association but also as a training teacher in Cross Academy - and a watchful eye on the emotional weak Zero Kiryu. The two of them quickly become friends after a series of tests (one being Kaito’s attempt to test Zero by inviting Yori-chan to an all Vampires party) and they partner up, in which they follow the commands of the Hunters Association like a team. Often Kaien will exclaim about how similar they are to their old mentor, Toga Yagari, and how he believes that they both possess Yagari’s attitude and problematic personality.
It's obvious how much Kaito cares about Zero the few moments he is shown, injuring himself even to protect his partner when they're guarding the Kuran Mansion.