Historians
The Traveler teams seem to receive extensive training, partly so they can all defend themselves but also to deal with misfired/killed team members. Each of them needs to be able to perform all tasks (even if not quite as well) so losing people won't cripple the whole team. The only position that seems entirely vested in one individual is that of Historian.
From what they've said about people being altered for the role, I think they do some kind of brain rewiring to give the historians what's called Superior Biographical Memory. This is the highest level of photographic memory, where a person literally remembers everything they ever do, see, or hear. You can ask them what they had for lunch on a random day thirty years ago and not only get the menu, but who they were sitting with, what everyone was wearing, their entire conversation verbatim, etc. So obviously this is one team member you really can't afford to lose. No one else can take over their job.