Born into the religious caste, more precisely the Third Fane of Chu'Domo, Lennier was brought up in a temple. There he studied the 97 dialects of the Minbari languages, history, alien cultures and mathematics. Martial arts were also an important part of his training.

Lennier arrived to Babylon 5 in early 2258. It was a great leap for a simple acolyte to become an attaché for a Satai. Because Delenn was a member of the Grey Council, Lennier was initially afraid to even look her in the eyes. She insisted, though, for her status to be kept in secret and for Lennier to treat her more casually. He had sworn to stand by her side and he did, supporting her even when other Minbari shunned her after she defied the Council and went through a transformation to become half-human, half-Minbari. His loyalty for her went beyond respect, though - he was, in fact, in love with her, although he knew he could never tell her of his feelings.

In short, Lennier was not only Delenn's aide, but also her trusted friend - he followed her to the Markab isolation area to help the people dying of drafa, saved her from a terrorist attack and often acted as her second in command on the White Star. Only Delenn's relationship with Captain Sheridan (who had during the war destroyed the Minbari ship Black Star and many of Lennier's family with it) made him feel a twinge of jealousy, but even then he swallowed his hatred and acted as Sheridan's second-in-command in several battles.

After the civil war broke on Minbar, Lennier followed Delenn to their homeworld to help her restore the peace between the castes. Due to a misunderstanding some of the crew on their ship attempted to assassinate Delenn, but Lennier was able to save her. He was willing to even die for her, and almost did, but in the end he only lost a part of his lung.

In early 2262 the death of Marcus Cole and Delenn's marriage to Sheridan made Lennier re-evaluate his life. He came to the conclusion that he could no longer act as Delenn's aide and left B5 to join the Rangers. Lennier's teachers soon began to suspect that something was wrong - he was trying too hard and his motives to join the Rangers were wrong. Lennier was in fact having problems accepting that Delenn was in love with another man and truly believed that the marriage between her and Sheridan was a mistake. His unrequited love had turned into an obsession. He believed that if he became a warrior like Sheridan, Delenn would fall in love with him.

During a break in training Lennier returned briefly to B5 to meet Delenn and to participate in Brakiri religious festival, the day of the dead. On the day of the dead the dead were believed to return from beyond death, and Lennier very much wanted to meet them and to ask them for wisdom. He got his wish as the ghost of Morden appeared to him and told him that he would betray the Anla-Shok and then die soon after.

Lennier's first mission as a ranger was to find evidence linking the Centauri to terrorist attacks on Interstellar Alliance shipping routes. Unbeknownst to Sheridan, Delenn assigned him to a whitestar patrolling on the borders of Centauri space. The crew of the whitestar managed to intercept coded transmissions, but before they could find out anything more about them, Sheridan heard about Lennier's mission and ordered the whitestar to return. Lennier didn't want to give up this easily and ran away determined to find the evidence. He succeeded, but instead of bringing peace into the ISA, his testimony started a war between the Interstellar Alliance and the Centauri.

Near the end of the war Lennier was escorting Delenn to Minbar when a group of Centauri ships intercepted them and badly damaged their ship. Assuming that they were going to die, Lennier finally managed to gather enough courage to tell Delenn about his feelings for her, but took it back when they were rescued.

Lennier's next meeting with Delenn made true part of Morden's grim prediction. Despite the fact that his training was in critical stage, Lennier took some time off in order to escort Sheridan and Delenn to their new home on Minbar. During the voyage there was an accident and some poisonous gas was released from the cooling system. While trying to rescue an unconscious ranger Sheridan was trapped to a room filling with the gas. Lennier witnessed the event and was just about to rescue both men when his feelings got the better of him and he ran off leaving the President and the ranger to die. He regretted his actions almost immediately, but by the time when he got back to Sheridan, it was already too late. The President had already been rescued. Realising what he had done, Lennier couldn't face Delenn, but took one of the fighters and ran away. Still, he contacted Delenn one more time promising that they would meet again after he'd made peace with himself.

Lennier died sometime during the next twenty years.