The castle is presently closed.
Aeleus to Ventus
Aeleus is seen wearing the same guard outfit as Dilan: a grey uniform with white cuffs and a red heart on the chest. There are blue hearts on the cuffs of both of his gloves. Aeleus's Axe Sword also appears much smaller than his Nobody's weapon, as well as being more reminiscent of an actual axe, rather than a combination of an axe and a sword. Aeleus looks nearly identical to his Nobody counterpart, except for some minor alterations.
Aeleus is shown guarding the palace of Ansem the Wise. He and Dilan meet Ventus, but keep him from getting inside the castle, telling him to leave before the monsters come out. Aeleus then spots a giant Unversed, and both guards begin to head after it as Ventus runs ahead of them.
As the two move to follow him, Even approaches and openly wonders what will become of the castle if they left their posts. Aeleus begins to argue, but Even assures Ventus could handle it because he is "special". As Aeleus questions this, Dilan recalls their job to guard the castle, and the two return to their positions at the door.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
About a decade prior to Kingdom Hearts, Aeleus was still one of six apprentices to the ruler and scientist of Radiant Garden, Ansem the Wise, alongside Braig, Dilan, Even, Ienzo, and Xehanort. Worried that 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. However, Aeleus 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 experimentations 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. Ansem was then banished to the Realm of Darkness by his apprentices.
Eventually, Aeleus and the other five apprentices dabbled too deep into darkness through their continued experiments, banishing Ansem the Wise to the Realm of Darkness. Aeleus is eventually attacked by Xehanort, who removes his heart as part of his original incarnation's plans for the X-blade. While Aeleus faded into darkness, his strong will allowed him to continue on as the Nobody Lexaeus, who joined the Organization as Number V; a Heartless was also produced.
Aeleus returns following the long time skip between the destruction of Lexaeus (and his Heartless). Coming to in Ansem's lab with Dilan, Even, Ienzo and Lea, he is one of the few who is fully stabilized. As Even and Dilan weren't he forced them to rest longer until they were better. He helped Ienzo search Ansem's study for clues while listening to him explain how someone is revived following the destruction of their Heartless and Nobody to Lea.