Xaldin was one of the six aprentices of Ansem the Wise. When they died they became Nobodies and formed the group know as Organization XIII. In Kingdom Hearts II he first appears in Beast's Castle. He's first seen talking to the beast trying to convince him that only anger will make him strong
and his desire to love and be loved in return was a pointless endeavor. Sora enters his room only to find out that he's gone wild crazy animal and Attack sora and his friends with cogworth's help they snapped him out of it. The beast accompanied sora to see Belle but wasn't in her room the talking dresser said Belle left to go look for beast. After they left the east wing they found Belle only to encounter a heartless it took a while because it changed from a black ball to a savage beast form in the second round of the battle after defeating it they find out that Xaldin was responsible for that heartless.
Sora and friends realized that Xaldin's Agenda was to corrupt the beast turn him into a heartless and take control of his Nobody that's born in the process but belle wishes that he changed a little more. Revisit Beast's castle Xaldin reappaered to interrupt belle and beast's dance at the ball room with his Nobodies as a division to take something the beast holds dear to him. After the battle with the dragon nobodies, the Beast discovers his enchanted rose is missing( the same rose that displays how much time he has left till his curse becomes permanent) the Beast shouts at Belle for not realizing its importance and Sora defends her . The Beast feels feels guilty he then tells Sora and Belle to leave the castle saying that after trying his best to change his beastly nature, he was only fooling himself so he might as well live like a beast since he is one.
Later Sora convinces him otherwise. Sora and the Beast head for the Main Hall to look for Xaldin but Xaldin finds them while carrying the rose. Xaldin then reveals his face and says "When the kingdom hearts is ours we can exist fully and completely" Then he calls the Nobodies to fight Sora and the gang defeat them and Xaldin leaves outside. The group head outside to look for Xaldin, meanwhile Belle sees the rose on her balcony and yells out to Beast she found it then Xaldin pops up from behind and takes Belle away to the castle gate The enraged Beast says to leave the castle Xaldin says he will but he'd rather travel light and makes Beast choose either Belle or the rose, but Belle elbow jabs Xaldin takes the rose and runs off.
Then finally the battle begins, Xaldin wielded six lances with the ability to control wind and the spears can transform into a dragon which made things harder. After a grueling battle, Xaldin was destroyed by Sora and the gang the Beast then asks Belle to stay with him. During the ending credits, the curse is shown to be broken, as Belle is dancing with the now human Beast.