Lennier is a cleric from the Third Fain of Chu'Domo, a prominent clan within the Minbari Religious Caste. He first arrives onboard Babylon 5 in the Earth year 2258 during the presentation of religious festivals from all the major alien races on board in the episode "The Parliment of Dreams". During his first encounter with Delenn, he refers to her as Satai, with his eyes glued to the floor. Delenn first orders him to look her in the eyes, a nod to the same order she received from Dukhat (considered by Minbari to be their greatest leader since Valen), when she did the exact same thing, since as the order was put, they "would forever be walking into things."
Delenn's second order to Lennier was to never again refer to her by her title "Satai", as it is the title given to those Minbari who served on their secretive Grey Council. Unbeknownst to almost anyone else on the station, she was there on Babylon 5 to observe Cmdr. Sinclair, since he was the first direct human contact the Gray Council would have, and would force Delenn to answer sensitive questions about her involvement.
Later in 2258, Lennier would assist Chief of Security Michael Garabaldi in the construction of a vintage Earth motorcycle. Lennier's interest in history, specifically that of Earth, led him to become somewhat overzealous and complete the motorcycle on his own, which Garibaldi was trying to accomplish himself. Despite some initial irritation, Garibaldi commended Lennier on his outstanding completion of the project, including the replacement of the gas tank with a Minbari fuel cell, and the two would race it down the station's main corridor.
Lennier was also involved in the "Deathwalker" incident on Babylon 5. When Warmaster Jha'Dur was assaulted, and war crimes charges were to be brought against her by the League of Non Aligned Worlds, Lennier cast a surprise "No" vote at the Council session while Delenn was out on other business, when everyone expected the Minbari to vote "Yes". The reason being, Jha'Dur was secreted away by the Windswords, a clan of Minbari Warriors that were considered incredibly zealous and militant, even by Minbari Warrior standards. With Jha'Dur's assistance, the Windswords developed terrible weapons during the Earth-Minbari War and if Lennier had voted "Yes", this revelation could have been brought to light and embarrass the Minbari. Lennier himself expressed great regret in casting the vote, but his hands were tied as a result of his orders.
His first interaction with Centauri Ambassador Londo Mollari came when Londo discovered much of Lennier's knowledge came from books and little real-world experience due to Lennier's time in temple. Lennier's skills in mathematics and probability would eventually lead to an altercation in a casino that caused extensive damage. In keeping with a little-known Minbari custom, Lennier took the blame for the damage to help Mollari save face, lying to Cmdr. Sinclair about it (it was a common misconception among most of the other races that Minbari don't lie under any circumstances).
Lennier later expressed some trepidation with Dellen's choice to enter a Chrysalis, and was also tasked during the time to ask Kosh a question, and upon returning to Delenn's quarters with the answer, said it was one word: "Yes."
In the beginning of 2259, Lennier was ordered by a representative of the Gray Council to approach the newly appointed Captain Sheridan and Cmdr. Ivonova about the real reason for the Minbari surrender at the Battle of the Line, the final battle in the Earth-Minbari War. He explained that, with Sinclair's capture, the Gray Council believed they had learned a terrible truth. They believed that Minbari souls were being reincarnated into Human bodies, and as a result of the strict Minbari taboo against Minbari harming one another, stopped the war immediately. The truth was, Cmdr. Sinclair was eventually to become Valen after travelling to the past with Babylon 4, since the Triluminary device used to interrogate him aboard the Gray Council's ship was calibrated to respond specifically to Sinclair's DNA. During this time, Lennier also would help to protect Delenn while still in the Chrysalis after the first officer of the Minbari War Cruiser Trigati, and a rogue member of the Warrior Caste, attempted to assassinate Delenn.
Later that year, it was discovered that a fellow member of Lennier's clan was implicated in the framing of Captain Sheridan. Many Minbari, including Lennier himself, had many reservations about Sheridan becoming the new commander of Babylon 5, for his sole victory during the war, in which Sheridan mined an asteroid field with nuclear weapons and detonated them when the Black Star (the Minbari flagship) was lured by a false distress signal, destroying it in the process, an act the Minbari considered highly dishonorable. Lennier admitted that several members of his clan were on the Black Star, but he put his reservations aside for the greater good when he realized that framing Sheridan, who was innocent the entire time, was considered an even more dishonorable act, and the shame from such a great dishonor as framing an innocent man for a murder he did not commit, would stain the honor of the entire Third Fane of Chu'Domo. Sheridan agrees to help Lennier and his clan "save face" with a creative retelling of the truth that satisfied everyone.
Lennier assists Delenn a few weeks later when the Markab's Drafa plague comes to Babylon 5 and wipes out the Markab population. It is eventually discovered most other sentient species (including Humans, Minbari, Centauri, and Narn) were genetically immune to it because they lack the cells that the plague attacks.
At the end of 2259, Lennier becomes worried about Delenn and defies her orders to not interfere in her interrogation at the hands of the Vorlon Inquisitor Mr. Sebastian, when he sees that Mr. Sebastian is literally torturing her to death. Lennier goes to Sheridan, the only person who Lennier felt could defy Kosh and stop Sebastian, and who has since developed feelings for her, and they pass the Inquisitor's tests. Unbeknown to most, Lennier also had feelings for Delenn, but kept them bottled up for the time being.
In early 2260, a series of bombings take place on the station, and Lennier sacrifices himself for Londo, pushing him out of the way of a blast at the customs area. Lennier is critically injured in the process and comatose as a result of head trauma. While recovering, Delenn and others visit him at his bedside. Londo spends a significant amount of time by his bedside, even telling him a few Centauri jokes. When Lennier emerges from the coma, he is told by Delenn that the Centauri government plans on giving him a medal for bravery, and Lennier quips that he should have stayed in the coma.
Later that year, Lennier and Delenn exchange information with Brother Edward, discussing Valen with Brother Edward, who, in turn, shares his experiences as a Roman Catholic monk. They did not know that Edward was a serial killer who was "mind wiped" and placed in a monestary as pennance for his transgressions.
Lennier is on board the White Star with Sheridan and Delenn in command as they head to the Jovian moon Ganymede to destroy a Shadow Vessel found there. He proves an able helmsman, translator and weapons officer, and they wind up using Jupiter's gravity to crush the Shadow Vessel, when Lennier himself was unsure if the White Star would survive a similar punishment. This action leads directly to Earth declaring martial law, and forcing Babylon 5 to eventually break away.
Shortly after this, Lennier sides with Delenn as they break the Gray Council for their failure to throw support behind Sheridan, and they assist Sheridan in breaking away from Earth with several Gray Council members' forces they successfully swayed to their side, though much to Lennier's heartbreak, soon after this, Sheridan first begins to openly express his love for Delenn.
During the rebirth ceremony shortly after Babylon 5 breaks from Earth, Lennier takes the opportunity to tell Marcus the secret he could tell no one else, in keeping with the ceremony. He admits to Marcus that he loves Delenn, but she cannot know. He says the love for her is a pure form of love, but subtly hints that he wishes to have it remain a secret since he doesn't wish to become a third wheel in the relationship between Delenn and Sheridan.
When Babylon 4 reappears in 2260, he along with many of the B5 command staff take the White Star to help hijack it and allow (now) Ambassador Sinclair take it to the past and fulfill his destiney as Valen.
A few months later, Lennier is seen attending a Baptist ceremony with Delenn and Sheridan, and attempts (hilariously) to keep up with the jubilant choir singing the upbeat hymn "And the Rock Cried Out No Hiding Place"
Shortly thereafter, Lennier plays a major role in one of the first engagements of the Shadow War, helping with fleet actions.
In 2261, Lennier, along with Ivanova, Lyta and Delenn, go to Z'Ha'Dum to find Sheridan, who went there against Kosh's instructions, blowing up the Shadow's main city in the process. Rumors swirled that Sheridan was seen falling into a deep chasm as the nuclear weapons he secretly brought with him on the White Star detonated. They were unsuccessful in finding Sheridan, and only due to Lennier's prudent thinking were they themselves saved (he programmed the White Star they were on to jump to hyperspace if Lennier failed to press a specific button on his console at a certain interval).
Lennier is also present during the Battle of Coriana VI, in which Sheridan and Delenn, with the help of the first of the First Ones, Lorien, drive our the Vorlons and Shadows from the galaxy after proving their guardianship over the younger races was a failure, and they themselves were the ones who had lost their way.
Later in the year, Delenn is summoned to Minbar to appear before her clan because her relationship with Sheridan has become known to them, and they are worried that such a union would pollute the Minbari genetic pool. Lennier stands with her in The Dreaming, a ceremony in which people revisit past events in their mind, and it is then that Delenn and Lennier learn a truth. She is a descendant of Valen (who they knew as Cmdr. Sinclair, a human transformed into a Minbari by the same triluminary that reversed the process in Delenn), and thus wasn't fully Minbari. Clan leaders eventually cite an old Minbari custom in which two people from warring sides would marry, and use the Earth Minbari war as such a conflict, permitting her to marry Sheridan.
As Sheridan begins the campaign to retake Earth from President Clark, Lennier travels with Delenn to Minbar again and see it in a state of civil war. Delenn's breaking of the Gray Council had unintended consequences. On their way there, members of the Delenn's own caste try to insert a toxic byproduct of the fusion reactor into the air to kill all on board, but Lennier discovers their plan, and manages to shut off the canisters leading to the air recyclers. This action cost him a part of his lung, and while recovering he saved face for the Religious Caste leaders while Delenn was in the room by claiming ignorance about the cause of it, but upon Delenn leaving the room, sternly admonished them for their short-sightedness, and nearly getting everyone killed for what eventually turned out to be something different.
Towards the end of 2261, as Sheridan is about to complete the coup against President Clark, Lennier discovers that the ambassadors, without Delenn's knowledge, have thrown their support behind Sheridan, and committed ships and material to his effort to retake Earth. They deliberatly kept Lennier and Delenn out of the loop in order to prevent the illusion of favoritism or influence due to Delenn and Sheridan being in a relationship.
In 2262, after the death of Marcus, who sacrificed his life to save a critically wounded Ivanova, Lennier decides to join the Rangers to help atone for the loss of his friend. Both Sheridan and Delenn express reservations about this, especially since Delenn only found out about Lennier's decision when a Minbari bureaucrat contacted her to discuss a replacement aide. Lennier eventually leaves the station th begin his training with no fanfare from anyone else.
During Lennier's training, he is stationed on board the White Star Maria. His training is under the watchful eye of Captain Montoya, who gives him high marks. He also helps fellow Minbari Ranger trainee Findell, and helps Montoya convince Findell to become a recruiter for the Rangers, with Montoya issuing a standing order to Findell to look all potential recruits in the eyes to see if they have what it takes to become Rangers.
Lennier returns briefly to Babylon 5 for the Day Of The Dead ceremony, and during the ceremony, is visited by Mr. Morden (the Shadow's agent among Humans), and Morden informs Lennier that he will betray the Rangers, which Lennier sternly rebukes at the time.
Later that year, Delenn secretly approaches Lennier without Sheridan's knowledge, and asks him to begin collecting intelligence in the sector his ship is assigned when it becomes apparent that trade routes are being attacked by a then unknown adversary, and there was no rhyme or reason to the attacks. Lennier nearly suffocates in his fighter when he picks up a distress call from one of the attacks, when he discovers it is the Centauri that are behind the attacks. When Lennier presents the evidence to the Interstellar Alliance council in person, Londo feels personally hurt and betrayed, even though he (Londo) was kept in the dark by his own government about the attacks. It was later discovered that the remnants of the Shadows servants that were inserted into elements of the Centauri government that were in fact responsible for this, but it was too late, and the damage was done. As a result, in a cruel twist of fate, Centauri Prime was bombed by Narn, Drazi and other forces, similarly to how Centauri Prime bombed the Narn homeworld only 2 years earlier.
During the bombing of Centauri Prime, while in hyperspace, Delenn's and Lennier's White Star is viciously attacked by overwhelming Centauri forces, leaving their ship crippled in hyperspace and both of them severely injured. Lennier finally admits his feelings for Delenn, just as other Centauri ships, not under control of leftover Shadow technology, come across their ship and save them in the nick of time. Delenn later pretends she was drifting in and out of consciousness when Lennier admitted his feelings, thus allowing Lennier to "save face".
After they both recover, and on their way to Minbar to help christen the new capital facilities of the Interstellar Alliance, the ship Delenn, Lennier and Sheridan are on breaks down, and Sheridan is shut behind a locked door filling with toxic gas. Lennier hesitates and runs off instead of pushing the button that would allow Sheridan to reach safety. In his shame, Lennier escapes the ship, since he realizes that he has fulfilled Morden's prophecy that he will betray the Rangers, by hesitating in saving Sheridan. Delenn and Sheridan try in vain to convince Lennier into changing his mind, but his decision was made, and he makes a tearful goodbye to Delenn.
During the next 20 years, Lennier travels around, and is involved in Earth's Telepath War, having been killed during a bombing.
When President Sheridan is dying in keeping with Lorien's 20-year prediction, Delenn raises her glass in honor of Lennier at Sheridan's final dinner in rememberance of friends long gone.
During Lennier's time on Babylon 5, he developed a close friendship with Londo's attache, Vir Cotto. The two would often meet up on the Zocolo when their free time allowed, and would vent their frustrations at being left out of sensitive proceedings for their respective governments, and when each left Babylon 5, the other would be among the first to wish them well on their journey.