The Brain

It might be cool to have this person be old, like Louis Wu in Ringworld. Maybe a ten or twelve term character in game terms. Extreme cases would be a brain in a tank or a guy in a low berth tube.

The Louis Wu idea is great, I seem to remember he was just bored having lived so long & therefore was up for risks and adventures. (I must re-read it).
Our guy could be similar, driven by travel, exploration and the search for knowledge (he is extremely intelligent so the knowledge accumulation is an obsession) with a 'what the hell I've had a great long life' kind of risk-taking attitude.
He is still very fit having led an adventurous lifestyle, & got the appearance of 65 year old by todays standards. No one knows quite how old he is but he hints at a few references which would make him a lot older than he could possibly be...
(he is either leading the others on or he has a few secrets!)

What about this guy being from a psionic institute? He could be a little lean on compassion and could at time be overly clinical. He could also be a non-psionic, but have a deep-seated fear/envy of them, that gives him a casual attitude to maybe even experimenting on them after a fashion?

Dr. Augustus Belaco

And since he's such a brain maybe he's manufacturing anagathics in the med lab while everyone else is busy or perhaps interesting psionic drugs...you know for kids.

And doctor of what field or fields?