"There is a hole...in your mind."
- – The Assassin to Sinclair
In 2257, as Babylon 5 neared its one year anniversary of going online, the Minbari assassin was sent to the station with the intent of assassinating the incoming Vorlon ambassador, Kosh. He had been sent by the Wind Swords to frame Sinclair for the murder and thereby disrupt the new and still fragile peace of Babylon 5.
He arranged to meet Del Varner to buy a changeling net on his way to the station, however, Varner was delayed. Instead, the assassin continued to travel on the Narn vessel and used a small pod to burn through the station's hull discretely. Once aboard, he quickly tracked down Varner, obtained the device but also killed him in order to assume his identity. The assassin was then able to take Sinclair's form and poison the ambassador through a "patch" he applied to the Vorlon's hand.
Having learned that Kosh had not quite died, but, rather, was being treated by Benjamin Kyle, the assassin took the form of Lyta Alexander to gain access to Medlab. He tried to discretely turn off the machines keeping Kosh alive, but Kyle noticed and interfered. The assassin left Medlab and, still appearing as Alexander, was discovered by not only Alexander herself but Sinclair as well. The assassin was tracked down by the energy signature given off by the device and Sinclair himself fought him, critically injuring him and proving his innocence at the same time. Rather than surrender, the assassin activated a bomb on himself, after telling Sinclair that he had a "Hole in his mind." Sinclair managed to avoid the blast, with some help from Delenn, who apologized for the actions of the Wind Swords.
There has been no in-universe explanation as to why the assassin's bone crest is shaped like that of a female Minbari.
Behind the scenes
While it was never stated in the show, the original arc memo, published in Volume 15 of the Babylon 5 Scripts of J. Michael Straczynski reveals that the assassin was to have been one of the crew of the Tragati