Kosh was one of the most important figures in the Army of Light - he was most likely involved with it as early as in Valen's time, given that Lorien seemed to know him. Some time before the Earth-Minbari war Kosh and Ulkesh acted as liaisons between the Vorlons and the Minbari (or more precisely their leader Dukhat).

Nearly two decades later, when the inevitable war against the Shadows was only a few years away, Kosh was assigned to Babylon 5. He arrived in 2257 and immediately there was an attempt on his life. Commander Sinclair was framed for it, but in reality the attempt had been arranged by G'Kar and a group of Minbari. In the aftermath of the assassination attempt two people, Doctor Benjamin Kyle and Lyta Alexander saw Kosh outside his encountersuit. Both of them were almost immediately called back to Earth. Lyta, who had scanned Kosh, had made a special connection with him. She became so spell-bound by Kosh that she was even ready to risk her life trying to get to the Vorlon homeworld. Eventually Kosh took Lyta as his aide and had her 'enhanced' so that she would be able to carry pieces of his consciousness in her mind.

It wasn't until late 2259 that Kosh appeared in public without his encountersuit when saving Captain Sheridan who had fallen from the core-shuttle. All those who had witnessed this saw a being of light - be it Valeria, G'Lan, Droshalla or an angel - each according to his or her beliefs.

Sheridan, like Lyta developed a special relationship with Kosh. The mysterious ambassador hardly ever attended to council meetings before Sheridan arrived to Babylon 5 and already in early 2259 he contacted the captain through a dream. Some time later (but before his angelic manifestation) Kosh revealed to Sheridan the truth about Anna Sheridan's death and began teaching him how to 'fight the legends'.

When the Shadows began openly attack the younger races in 2260, Sheridan turned to Kosh for military assistance. At first Kosh refused, but Sheridan's persistence changed his mind. However, the Vorlon involvement was almost immediately revenged by Mr. Morden, whose associates killed Kosh. Still, not all of Kosh died - a piece of his consciousness moved to Sheridan's mind. Later that year, when Sheridan was trapped in Z'ha'dum, Kosh told him to jump into a deep abyss knowing that this would bring the Captain to Lorien.

After Lorien had brought Sheridan back from the dead, Kosh helped the Army of Light one more time by killing Ulkesh who had succeeded him as the Vorlon ambassador. Even this wasn't the last of Kosh - a year later, during the Brakiri day of the dead, he sent Sheridan a message ("When the long night comes, return to the end of the beginning") through Captain Lochley's dead friend Zoë.