Kingdom Hearts Birth by Sleep
Dilan appears with Aeleus, both guarding Ansem's castle, Hollow Bastion, in Radiant Garden. Ventus first encounters them while he is looking for Mickey Mouse, but they refuse to let him in saying that the castle is closed. Dilan suggests that Ven should go home before the monsters get him. Aeleus then spots an Unversed, and both guards are about to attack it. Ventus runs ahead of them and begins chasing it, despite the guards' protests. They were about to go after Ven, but Even comes and openly wonders what will become of the castle if they leave. Aeleus and Even begin to argue, but Dilan reminds Aeleus that their duty is to protect the castle. Even then tells them that his Lordship wants to see them and both guards head inside.
Later, Braig brings Dilan and Ansem to an amnesic Terra-Xehanort, along with Aqua's Keyblade and Keyblade Armor that had been left with him. Terra-Xehanort can only remember his possessor's name: Xehanort. While Braig takes "Xehanort" to Hollow Bastion, Dilan carries the armor and Keyblade for Braig.
 Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
About a decade prior to Kingdom Hearts, Dilan was still one of six apprentices to the ruler and scientist of Radiant Garden, Ansem the Wise, alongside Braig, Even, Aeleus, Ienzo, and Xehanort. Worried the Darkness in people's hearts would eventually consume his world, Ansem began performing tests on Xehanort's heart. Eventually, he created a laboratory in his castle's basement for such experimentation, by the urging of Ienzo. However, Dilan and the other apprentices had their own intentions for the laboratory, and began conducting inhumane experiments on the human heart and the darkness within it. Such experimentation led to the creation of Heartless. Ansem quickly discovered their secret experiments, and immediately ordered them to stop. One day, after being advised to review their research data by King Mickey, Ansem found that the effects of their experiments had become irreversible. Eventually, Dilan and the other five apprentices dabbled too deep into darkness through their continued experiments. Dilan, along with the others, was consumed by the darkness, producing a Heartless. However, the apprentices all had strong wills, and so Dilan also became his Nobody, Xaldin. The six Nobodies formed Organization XIII, and banished Ansem the Wise to the Realm of Darkness.
 Kingdom Hearts 3D: Dream Drop Distance
Dilan returns following the destruction of his Heartless and Xaldin. He wakes up in Hollow Bastion with Even, Ienzo, Aeleus and Lea. Having not fully recovered, he rests with Even as Ienzo and Aeleus search Ansem's study.
Dilan is a tall, broad-shouldered man that wears a dark slate grey guard uniform which is adorned with a curled heart symbol, and white gloves. The front of his jacket sports two rows of gold buttons, silver highlights, and a red and white heart like the one on his gloves. He also wears a belt and knee-high boots, both of which are black. Dilan's hair is black and worn in long dreadlocks. He keeps most of his dreads tied into a low ponytail, but one dangles over the right side of his face. He also has thick sideburns, (though they do not hook over his cheeks like Xaldin's do), thick eyebrows, and violet eyes.
Dilan wields a single lance with silver edges, a blue head, and a handle that alternates from violet to silver. The curled heart symbol is incorporated into the head of the lance.
While Dilan appears to bear similarities in personality to Xaldin, given his rather strict nature, his character seems to be much more benevolent and noble than his Nobody, demonstrated when he was willing to defend Radiant Garden from the Unversed and expressed genuine concern for Ventus's well-being after the latter volunteered to fight the Unversed alone. In fact, Dilan was only stopped from following Ventus by Even ordering him to return to his post.
A strong possibility is that Xaldin's hostile attitude towards Axel shown in Kingdom Hearts II Final Mix is derived from Dilan's confrontation with Lea at the end of Kingdom Hearts Birth by Sleep, whom he throws out of Hollow Bastion along with Isa for trespassing.