Lennier is appointed to Delenn as an assistant, Still I don't really know what he does!

Well when Delenn and Sheridan get married he moves back to Minbar and Well I would say Sulk.

During this time I think, when Delenn and Lennier where working together, he began to have feeling for her, and when Delenn and Sheridan got married he could not stand not to have her so he move away.

During the last seasons I think Sheridan is trapped in a room where the poison air was filling, then Lennier turned up and, well he ran off. He wanted him to die basically. But Sheridan and an officer that was trapped in the room got out.

Lennier stole a Minbari fighter and disappeared, sent a lest message to Delenn and that is it..

Really that is all about this person, but if I learn any more I will add it.