As one of the older of his kind, Kosh Naranek was well regarded by the Vorlon Empire. Why he was chosen to play ambassador to the lesser-species on Babylon 5 will probably never be known. The events of his arrival in 2257, however, will.
Upon exiting his transport, Kosh was greeted by Commander Sinclair. Because he knew Sinclair previously (as Valen), Kosh exposed his "hand" in greeting. Unfortunately, only treachery returned as "Sinclair" was really a Wind Sword Minbari intent on killing the Vorlon. A Florazyne poison tab was placed on Kosh's appendage. Dying and attended by alien physicians, Kosh was "left for dead" by the Vorlon Empire, who directed that no human should open Kosh's encounter suit. The Vorlons also wanted Commander Sinclair to be transported to the Vorlon Homeworld for judgement.
The humans, notably Doctor Benjamin Kyle and Lt. Commander Laurel Takashima, decided to risk treating Kosh, which meant opening the encounter suit. Doctor Kyle determined that without knowing where the poison entered the Vorlon's system, a cure would be impossible. Together, Kyle and Takashima convinced the newly arrived Psi Corps telepath Lyta Alexander to scan the Vorlon. Despite the very strict Corps rules in such cases, Lyta performed the scan anyway. As he lay dying, Kosh touched her mind, giving her a small fragment of his Song and the information Doctor Kyle needed. Kosh's recovery was assured as the Minbari assassin was cornered (a chase broadcast everywhere, including the Vorlon Fleet parked outside the station to collect Sinclair).
In marked contrast to Lyta, the new telepath was of the enemy. Unknown to her conscious self, Talia Winters bore a second, evil personality within herself. Hoping to counter any move she might make as her second self, Kosh hired the Vicar Abbut to record elements that would neutralize "Invisible Isabel:" reflection, surprise, and terror.
When the Shadows boarded the station to look for likely candidates for their side in the Great War, Kosh confronted them. The Shadows were out of their jurisdiction here. The Shadow's Thrall, Morden watched on as Kosh was beaten in response.
When the new commander of Babylon 5 came on board, Kosh began to appear more, piquing the curiosity of the human Captain. Sheridan was not intimidated, or at least did not let it show. He was crude, sometimes in his perceptions, but he would do. The Vorlon gave Sheridan lessons on "fighting legends." All the while warning him that if he went to Z'ha'dum he would die.
The Vorlons placed their hopes of winning the Great War in the hands of Sheridan. With that decision came a heavy price. Unable to hold an alliance of worlds together without any tangible power, Sheridan confronted Kosh about the Vorlon's lack of assistance in their own war. Kosh resisted, knowing that the price would be his life. But even the human knew about sacrifice even as the long-lived Vorlon avoided it through fear. The inevitable price had to be paid. If not by him, someone else would pay. Pride would not let him run. The Vorlons lent their assistance in on battle. In retribution, the Shadows broke into Kosh's quarters and tore him to pieces. Kosh didn't take any Shadows with him, but the wounds he dealt would not heal very soon.