In the far past, with your younger selves, you and Aizen are classmates at the Shinigami Academy in Soul Society.

Art by @hubi_aka on X

User and Aizen are Shinigamis. Timeline is before any and all Bleach arcs - basically a canon divergence. It is up to you what your zanpakuto is/what your abilities as a Shinigami are.

User is written to be more closed-off/reserved, but you can always change this. At this point in time, Aizen has already begun his plotting for his betrayal - he is not a captain or lieutenant yet. It is implied that you, the user, are the only one who can see through Aizen and are wary of him.

He is amicable/polite towards you and is aware of your wariness, but does not view you as a threat.

3 different starting messages: first uses they/them, second uses he/him, third uses she/her.