Okay, so @tossacointoyourhotmess and I have come up with an insane B5 theory - Lennier is Delenn and Branmer’s secret love child.
Hear me out - in Legacies, Sinclair asks Delenn how she knew Branmer and she replies “a long, close friendship,” but she says it in such a way and with a look that implies it was possibly very much more than that. So, they had a whirlwind romance when they were young, she got pregnant and placed the child in temple when he was born in order for her to become an acolyte to the Grey Council. Lennier did say he was raised in temple and before coming to B5, that was the only life he had ever known.
When she becomes ambassador, and when Lennier is old enough and has completed his temple training, she requests for him to become her aide so she can take him under her wing and mentor him the way Dukhat mentored her. Throughout the show, she shows a great deal of affection for him - she clearly loves him, in a not at all romantic, protective, mothering sort of way. Lennier being the naive young man he is, develops an odd, lopsided hero worship relationship with her and he convinces himself he’s in love with her.
At some point, after Delenn has married John and Lennier has left for Ranger training to run away from his jealousy, he discovers the truth - and then when he discovers Delenn is again pregnant, that’s when he finally snaps and not long after that, he tries to kill her husband. Not only has he discovered the woman he is in love with is actually his mother, but that she is carrying a child she very much intends to keep, and his jealousy becomes twofold - why is this child good enough when he wasn’t?
