Kai wasn't always a demon. Kai changed his name 5 year's ago from Joe.
Joe was a Half-anthro, with human skin and panther ears, claws and paws. He has no recollection of his parents, and his earliest memories are of when he was 8, a year before he met Alex. He was an extraordinary kid, with a natural gift for fighting and swordsmanship. Alex found him one summer, and they made best friends. Joe came to live with Alex at the castle, and got enrolled in the booster program at Alex's boarding school. He was there in the room with Alex when King Greatwolf died, as the King was possessed by darkness and attacked the two boys. Joe fought him off while Alex froze, but was overpowered and Alex had to finish his father off. Alex and Joe grew distant after that, which didn't help as Joe completely dismissed the event like it didn't even matter.

The event that led to Joe changing his name happened while he was that while assisting Takashi, he was trapped between this world and the plains of Hell. Joe's shadow and his light were separated into two deities. Kai was actually the light deity, which lost when the two fought for dominants.
After losing, the shadow spared the light, only to enslave it for a short time after they reclaimed their material bodies. Eventually the shadow, which named itself Caine, release Kai, but Kai never felt secure with himself after that encounter. Kai desperately searched for a way to gain more power then Caine so he could defeat him when they fought again. This lead him to find demons, and he took the trials to become a demon himself. Although he has a demon's body, he isn't considered a full demon because he still has a mortal's soul. This qualifies Kai as a 'Demon-Blood'
Kai traded his real eye for demonic power, and replaced it with a phantom that was trapped inside a relic. This phantom eye can see through illusions as long as Kai knows they exist. It can Also detect life and detect magic, but again if Kai isn't aware that the life or the magic exists, the eye won't know that Kai can't see it.
Kai hasn't changed his personality much, he is kind of an idiot. . He's stubborn, and often Kai rushes into battle refusing to form a strategy. He's fun loving, devious and never takes anything seriously except fighting. Kai is still learning how to control his new body and powers, but being a Wrath Demon of War, he has the capability to bring this world and all the other worlds to their knees.

Kai's Swords:
Kai's personal sword is called Shinto. It was made when Kai turned into a demon and is the symbol of his demonic power. It is imbued with part of his soul and has part of his power and Personality within it. This sword defies control and people who are 'lawful' by nature tend to get hurt more by it. Shinto can use one of two mediums to enter this world through Kai: the quick way through blood (even Kai's) or the long way through shadows.
Length: 36 inches
Weight: 9.46 lbs

Threasha is a cursed sword Kai found in his travels to find demonic power. It's bound to Kai, and anyone else who tries to wield it will be cut by her. It was made from the bones of a very ancient demon god. This god was materialized into this existence, and was only stopped when trapped between realities. This god had it's name permanent banned from this reality, so that no one may summon it again. Threasha is the embodiment of this God's rage, and can turn blood into sharp blades, creating a title wave of red liquid that cuts through steel and bone growing as it fells it's enemies. Threasha is broken however, and cannot sustain this state long and sometimes shatters. When it shatters it uses some of Kai's life force to repair itself.
Threasha is female and sometimes takes on a material body of it's own.
Length: 41 inches
Weight: 13.1 lbs.