Initially Lennier seemed shy and naïve around other races and in the busy world of the station, and the ingrained habits of the temple made him deeply respectful of all especially Ambassador Delenn. Although he had been given her instruction to "look up" when he spoke to her, in contradiction of Minbari custom, he found this difficult. "Understanding is not required, only obedience" is what his temple training had taught him.

     Lennier had an early run-in with station security over a misunderstanding in a card game, where his quick thinking saved Ambassador Londo Mollari of the Centauri from embarrassment. He also was seen disrupting life in the Zocalo when he and Security Chief Michael Garibaldi took a ride on a 20^th Century Earth motorcycle in the reconstruction of which he had been material, including installing a Minbari power source to avoid the pollutants of a 20^th Century petroleum engine.

     Lennier quickly became invaluable to Ambassador Delenn, not only in the political world of an ambassadorial aide but also in the ceremonies of the Religious Caste to which they both belonged. He helped her in a demonstration of the Re-Birth Ceremony, the Minbari contribution to Commander Sinclair’s celebration of the dominant religions of the races resident on Babylon 5. When the time came for Delenn to enter the Chrysalis in fulfillment of prophecy, Lennier watched over and protected her during her time of transition. It was to Lennier that Delenn entrusted the truth about the Minbari surrender at the Battle of the Line, and it was he who revealed this to the new Captain of Babylon 5, John Sheridan, when a Minbari threat to the station made this necessary.

     It was Lennier who was present and summoned Dr. Franklin when the transformed Delenn emerged from the Chrysalis, and it was he who presented the now half Human half Minbari Ambassador to the Station Council. Lennier’s loyalty was proved beyond doubt in his standing by Delenn, as her transformation was not at first accepted by Humans nor, especially, Minbari who saw her as a freak and "no longer one of us". When Delenn was expelled from the Grey Council, Lennier chose to remain with her in spite of Delenn’s having warned him that it might cost him his advancement and standing in Minbari society. Lennier also stood against some of his own clan members and risked dishonor when a few plotted to discredit Sheridan over his killing of a Minbari on the Station. Sheridan however proposed a compromise which saved face for Lennier and his clan.

     Lennier accompanied Delenn in the brave but futile attempt to bring help and comfort to the dying Markhab, entering their isolation zone without knowing if they also might be infected. When Delenn was interrogated by the Vorlons’ inquisitor, Lennier went against her specific instructions in asking Captain Sheridan to intervene - an act which probably saved her life.

     The development of the first White Star ship saw Lennier develop skills as a navigator, pilot and fighter. He served on the bridge of this and the later ships of the White Star Fleet throughout the troubles which led to the Babylon 5 break with Earth and subsequent war, and also throughout the Shadow War.

     The kidnapping of Delenn by Night Watch members loyal to President Clark’s Earth Government made Lennier confront his feelings of love for her and then confess the same to Ranger Marcus Cole as part of a Re-Birth Ceremony Delenn had begun before the kidnapping. Lennier convinced himself his love was "higher, purer, nobler" and that he could accept Delenn’s deepening relationship with Captain Sheridan. When Delenn was appointed the new head of the Rangers Lennier disobeyed her implicit instructions and persuaded Marcus Cole to stop Neroon who was determined to prevent Delenn’s installation as Entil Zha.

     Lennier again was Delenn’s support when Sheridan was presumed dead at Z’Ha’Dum, and it was he who asked Dr. Franklin to intervene when, in keeping with Minbari tradition, Delenn’s fasting over the loss of Sheridan threatened her own life. And it was Lennier’s quick thinking in programming the ship to take them away if he failed to give a timely signal which saved Delenn, Susan Ivanova and Lyta Alexander when they went to Z’Ha’Dum hoping to find Sheridan alive.

     Lennier voluntarily accompanied Delenn in the Dreaming, when her clan on Minbar questioned her planned marriage to Sheridan. Even though the Dreaming revealed it had been Delenn’s order which began the Earth-Minbari War, he stayed with her and heard the revelation that she was a Child of Valen. It was Lennier who obtained evidence from the Grey Council Archives of the prevalence of Human DNA among Minbari through the descendants of Sinclair/Valen, enabling Delenn to continue her relationship with Sheridan.

Throughout the worsening situation on Minbar, which would culminate in the Minbari Civil War, Lennier was at Delenn’s side. It was he who first realized who the Drakh were and piloted the ship to safety before turning and destroying the Drakh mothership. He later saved their ship when a toxic coolant leak was deliberately caused, protecting Delenn from the truth that the sabotage was done by members of their own Religious Caste.

     Accompanying Delenn to Minbar itself and the war, Lennier was with her when Delenn placed herself in the Star Fire Wheel, ready to sacrifice herself for her world and people. It was to Lennier Delenn had entrusted her instructions before entering the fire. Although she did not die through the intervention and self-sacrifice of Alyt Neroon, Lennier had obviously been designated her successor as leader of the Religious Caste and leader of Minbar, as the Religious Caste had ended the war.

     Lennier accompanied Delenn and the combined fleet of ships in the attack on Earth that finished the dictatorship of President Clark. In the aftermath the non-aligned worlds were persuaded by Delenn and Lennier to join with the Minbari, Centauri, Narn and later Earth to form a new Interstellar Alliance with Sheridan as President. Delenn together with Centauri Ambassador Mollari and Narn Ambassador G’Kar would serve as Advisory Council. The White Star Fleet under the command of the Rangers became part of the new Alliance.

     Following the marriage of Delenn and Sheridan, Lennier no longer felt comfortable as Delenn’s aide and asked to join the Rangers. Delenn reluctantly let him go as he assured her it was "the calling of my heart". She and Sheridan were both aware of Lennier’s true but undeclared feelings for Delenn. Lennier threw himself into Ranger training in an intense manner that caused concern to his teachers, especially Sech Turval.  Lennier’s face was scarred in a Denn ‘Bok practice when another trainee moved more quickly than expected. Lennier returned briefly to Babylon 5 for the Brakiri Day of the Dead ceremony during which he was confronted by Mr. Morden who told him two truths: that Delenn would never love him as he loved her and that Lennier would one day betray the Rangers.

     Delenn secretly recalled Lennier to the Station and assigned him to White Star 27 under Captain Montoya., ostensibly as a trainee Ranger but actually to monitor signals which would prove who was making attacks on unarmed Alliance shipping. During his time on White Star 27 Lennier saved another trainee and helped him face the real reasons he had joined the Rangers – something Lennier had not yet faced himself.

     Delenn had not told Sheridan of Lennier’s secret assignment, and when Sheridan re-called White Star 27 on learning of it Lennier deserted in a small Minbari fighter to continue his mission. Almost dead from lack of oxygen, Lennier latched on to a passing Centauri ship from which he siphoned a new air supply. The ship carried him into a merciless attack on civilian transports. Lennier’s onboard camera recorded the attack. He was recovered by White Star 27 and brought to Babylon 5 where his evidence led to the expulsion of the Centauri from the Alliance and the beginning of hostilities between the Centauri and other Alliance worlds.

     Lennier was still with Delenn when she left Babylon 5 to try to persuade the Minbari Grey Council to work with Earth in developing a more powerful ship combining Earth, Minbari and Vorlon technology. On the way their White Star was attacked by Centauri ships and left adrift in hyperspace with most of the crew dead. Believing they were about to die when the Centauri ships returned, Lennier admitted to Delenn that he loved her. The Centauri ships however had been sent to rescue them by Londo Mollari.

     Lennier returned to his Ranger training on Minbar, only returning to Babylon 5 to assist Delenn and Sheridan on their move to the new Interstellar Alliance Headquarters on Minbar. During the journey a poisonous system leak led to a section of the White Star being sealed down, trapping Sheridan and a Ranger he had been trying to rescue inside. Lennier came upon them and in a moment of weakness left Sheridan to die. Regretting his action almost immediately he returned, but too late as Sheridan had managed to break free and the two had been rescued. Appalled at his actions Lennier took a fighter and abandoned the ship, cutting off Delenn’s attempt to call him back. A search of his belongings at the Ranger training center on Minbar revealed his personal journal where he had written of Delenn’s marriage with Sheridan being a mistake.

     Lennier sent one last communication to Delenn, telling her how sorry he was and that he would have told Sheridan himself but he was too ashamed. He asked her to be happy and hoped that someday she could forgive him. Lennier disappeared.