Tam Fairbrother is an exceptionally good-looking young man who is part of the anti-Pirate squad on the Celestial Omnibus. A member of one of the lower-level witchcraft circles in Britain, he excels at theater and keeps the rest of the squad in stitches while they wait to embark.
Tam's further significance in the novel is that he is the subject of an autopsy on Arth, where no one can figure out what killed people on the Omnibus. Healing Horn's Brother Edward discovers that Tam is nearly a dead ringer for one of the cadets on Arth -- young Josh the centaur. This gives rise to a theory about the impact of analogues upon inter-world travel, which promises to be a useful and perhaps lifesaving discovery.