Kagami hails from the land of the dead, a lost and deceased member of the Kaguya clan. He was reanimated by stray magic and thus, he can sustain his body as if he were alive. He is not however.
Kagami died in the KG wars, falling to a sharingan user in battle. The Uchiha was unnamed, but Kagami's death was decided with the Uchiha's appearance. The battle left Kagami with his arm almost bare, stripped of most of its skin and muscle. The Uchiha toyed with him a little more, laughing at his pained flailing of his almost useless appendage. Kagami's pride wouldn't let him submit, so he kept fighting. He kept fighting until the most bitter end, with almost no blood left and barely the strength to stand.
It wasn't long before Kagami succumbed to the cold embrace of death.
With his last breath, he spoke the words "Damn this clan... And damn... This... Arm..."
The resentment and anger Kagami left in this world was enough to curse his left arm.
In death, he used his curse to absorb much strength from the legions of undead, slowly moving through the underworld, making a name for himself. But when he was struck by that magic and brought back to the physical plane, that only gave him more strength to make his way through the underworld with. It gave him his bones back, and with it, his lust for revenge on the Uchihas. Who was that man who ended his life?
It didn't matter to him, all that mattered was getting back to the living realm and continuing his life.
Which is what he did.
Outside of his ulterior motives, Kagami only wants to keep living his life that was ended before its time. He is a nice person, always ready to lend a hand. He does have quite the temper though...