Kingdom Hearts Birth by Sleep
Aeleus appears with Dilan, both guarding Ansem's castle, Hollow Bastion, in Radiant Garden. 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 an 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 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, Aeleus and the other five apprentices dabbled too deep into darkness through their continued experiments. Aeleus, along with the others, was consumed by the darkness, producing a Heartless. However, the apprentices all had strong wills, and so Aeleus also became the Nobody Lexaeus. The six Nobodies formed Organization XIII, and banished Ansem the Wise to the Realm of Darkness.
Kingdom Hearts 3D: Dream Drop Distance
Aeleus is restored following the demise of his Heartless and Lexaeus. He awakens within the Heartless Manufactory with Dilan, Even, Ienzo and Lea, and soon realizes that Braig and Isa are mysteriously absent. As Dilan and Even are still tired and unstable from their restoration, Aeleus has them rest until they've finally stabilized.
Kingdom Hearts III
He and Dilan later search the town for Even, who had gone missing. However, they are unable to find him. Aeleus reports this news back to Ienzo, who is currently discussing replicas with Riku and the King.
The night before the second Keyblade War, Aeleus and Dilan rush into the lab to see Ienzo with their former master Ansem the Wise, who has been returned to them by Demyx. As Ienzo apologies to Ansem for the apprentices' betrayal, Aeleus comforts a remorseful Dilan.
Following the final battle with Xehanort, Aeleus and Dilan in Radiant Garden watch as Naminé comes out of the lab after her examination by Ansem the Wise, Even, and Ienzo, when Riku arrives on a Gummi Ship to pick Naminé up.
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.
Like with Dilan, Aeleus appears to be much more good-natured and noble than Lexaeus, although their outward personalities seem similar. He shows concern for Ventus when the latter volunteers to fight Trinity Armor alone, and is only stopped from following Ventus by Even, who orders him back to his post.
In Kingdom Hearts III, Aeleus appears to be remorseful of his actions as a Nobody, judging by his reluctance to speak with Sora and how he aided his friends in their quest in defeating Master Xehanort. Further, Aeleus indeed regretted how he and his fellow apprentices betrayed their teacher, Ansem, the Wise and thus, was able to make amends with his former master.
Notes and references