Member of the Third Fane of Chodomo clan (There All the Honor Lies), serving as Delenn's aide and secret admirer (Ceremonies of Light and Dark).   Delenn herself finds that she cannot live without his wisdom and guidance (Rumours, Bargains and Lies).

Among his many achievements are motorcycle maintenance (Eyes), bar-room brawling (The Quality of Mercy), and piloting the Vorlon-Minbari hybrid technology White Star ships.  Although Minbari technically do not lie, Lennier finds himself having to do so several times to save the honour of others (The Quality of Mercy, There All the Honor Lies, Meditations on the Abyss).

In early 2262, in the wake of Delenn's marriage to Sheridan, he decided to leave her employ and join the Rangers (The Very Long Night of Londo Mollari).  During the Brakiri Day of the Dead ceremony, Morden appeared to him and told him he would one day betray the Rangers (Day of the Dead).   During his Ranger training, he found evidence indicating the Centauri responsible for attacks on the Interstellar Alliance shipping lanes (Darkness Ascending).  Despite the fact that he denied this prophecy, it eventually comes true after his love for Delenn causes him on impulse to leave Sheridan for dead (Objects at Rest), driving him away from her to regroup his thoughts and feelings before he feels able to return and beg for forgiveness.

His final fate his ambiguous (Sleeping in Light).

Played by Bill Mumy, who played Will Robinson in the original TV series of Lost in Space, and also appeared in the Star Trek: Deep Space Nine episode The Seige of AR-558.