Representative of the ancient Vorlon race, Kosh remains inside an encounter suit to stop the other alien races from recognizing him (In the Shadow of Z'ha'dum). Kosh was with the Minbari during the Earth-Minbari War, after contacting Dukhat about the possible re-emergence of the Shadows on Z'ha'dum (In the Beginning).
Shortly after arriving on Babylon 5, Kosh was attacked, his encounter suit opened by Dr Benjamin Kyle, and telepathically contacted by Lyta Alexander (The Gathering). He made a recording of Talia Winters (Deathwalker), encouraged Delenn to go ahead with her transformation (Chrysalis), and eventually trained Sheridan for the ordeals he was destined to face (All Alone in the Night, Hunter, Prey, There All the Honor Lies). He sent for an Inquisitor from the Vorlon homeworld to be certain that Delenn and Sheridan were the right people for the job (Comes the Inquisitor).
He finally emerged from his encounter suit to save Sheridan's life, to the amazement of all onlookers, appearing to each one of them as a figure from their respective worlds' mythologies - except Londo Mollari (The Fall of Night). When a physiologically altered Lyta Alexander returned to the station from Vorlon space, Kosh utilized her as a "carrier" for his conscience, so he could observe events aboard the station surreptitiously (Passing Through Gethsemane).
Kosh was eventually killed by the Shadows when the Vorlons became more actively involved in the War (Interludes and Examinations), but leaving a "piece" of himself within Sheridan's mind, who remained with Sheridan when he went to Z'ha'dum (Z'ha'dum; see also the novel Invoking Darkness).
A new Vorlon Ambassador, also naming himself Kosh ("We are all Kosh" - however, according to Invoking Darkness this new Vorlon's proper name was Ulkesh), arrived on the station to replace him (Walkabout) but, when the Vorlons began to openly attack the Shadows in force, he was killed by the station crew and the remaining piece of the original Kosh (Falling Toward Apotheosis).