Kai is Enjouji's half-brother, having the same father, the yakuza boss, but different mothers, both of whom haved passed away. Having a spoiled yet emotionally difficult childhood, Kai is somewhat of a malcontent and dislikes Kei or at least has an extreme sibling rivalry with him! However, he has a constant although a bit ill-defined relationship with his father's second-hand man, Masanori Araki. Initially he is very attracted to Ranmaru, serving as an additional hurdle for our main protagonists to overcome. But Kai is quite young and is still going through maturation in this series' storyline.

source: http://www.angelfire.com/anime4/neesachan/synopses/kizuna/index.html#anchor7344199</a>