Well, let's say I have a feeling we are destined to cross paths again.
Even to Ventus
Even's appearance matches his Nobody's appearance exactly, except for the differing outfits and his eyes, which are a brighter shade of green. He wears a long white lab coat, grey vest, white collared shirt, and a purple ascot. He also seems to wear black pants and silver lined boots identical to those worn by Organization XIII members.
Even first appears as he approaches Dilan and Aeleus, and openly wonders what will become of the castle if they left their posts. Aeleus begins to argue, wanting to chase after Ventus and an Unversed, but Even assures them that 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, and Even stands in place as he looks on at Ventus trailing an Unversed. He takes note that Ventus has no darkness in his heart, and after commenting that it may have been removed, he deems it impossible and turns to walk back to the castle.
Later on, Even looks around for Ienzo, calling his name. He finds him walking towards Ventus, and he approaches the two, reminding Ienzo not to leave because it is dangerous. He then turns to Ventus and thanks him, explaining that Ienzo's parents have passed away, and that he is in charge of raising him. Ventus then asks if Even has seen his friend, to which Even replies he had seen him in the castle garden. As Even and Ienzo are about to leave, Even states his belief that they are fated to meet again someday.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
Worried that the Darkness in people's hearts would eventually consume his world, Ansem and his apprentices began performing tests on Xehanort's heart, which Even showed particular interest in. Eventually, Ansem created a laboratory in his castle's basement for such experimentation.
However, Even 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. Ansem was then banished to the Realm of Darkness by his apprentices.
Even was eventually betrayed by Xehanort, when he removed his heart along with the other apprentices's. While his heart went on to become a Heartless, Even's strong will enabled him to continue on as a Nobody, which was given the name Vexen. He went on to found the Organization with the Nobodies of his fellow apprentices.
Even returns after the defeat of his Nobody, Vexen and the unseen destruction of his Heartless. Unlike Lea, Aeleus and Ienzo, Even is unstable after being restored to human form, being forced to rest, along with an equally unstable Dilan, until he stabilizes.
Even disappeared from Radiant Garden, becoming Vexen again in order to rejoin the Real Organization XIII, as their thirteenth vessel, so he can pursue his research into the Replica Program. He was, later, replaced by Xion and became a reserve member with Demyx. However, Even's true motives were of penance for his past actions.