Enju is Kai's present name, Kai is his former name in the past. He actually taken interest in Sakura her younger sister later in the series. Enju is one of the Moon Kingdom's alliances or known as 'Demons from the Moon Kingdom'.
Enju the long lost brother of Sakura, Enju has a personality which is more "nice" on the outside, but "bad" on the inside, more of a *yandere type (but not that scary) and has a 'sister-to-brother complex' an unrequited love in the end which Enju died.
Enju has silver hair and blue eyes. He also has a stone on his forehead symbolising he is a youko and is no longer Kai.
Enju, in the end has a 'sister to brother complex', he only seemed nice to his group members, and ONLY to Sakura, Rurijou is merely just a replica of Sakura, but actually likes Sakura more than Rurijou. And he would destroy anyone who gets in his way of getting Sakura, saying they 'brainwashed' her. But in truth, he only wants to help Sakura out.
Before his awakening, Kai found himself imprisoned in a water prison for 5 years. When he broke out, he searched for Sakura, ordering the chief and Oumi to follow his orders, just not letting them die or suffer. His first appearance is when the chief sent a snake out to chase Sakura, but in the end Enju killed him, and took back Sakura when the time Oumi is killed in Oumi's mansion (Sakura blossom leaves field). And thus they took Sakura into a hidden base which is exactly like the Moon Kingdom. Setting one out until all of his plans were doomed, as so his members. Enju was killed while protecting Sakura from being slashed by Chizakura, wielded by Princess Kaguya.