Eberron: Equilibrium's End
Bernard Bernoulli, a nerd suffering from cowardice (he runs away from Green Tentacle until another character makes friends with it). He has the most skills of any character in the game, as he can disassemble the radio in the den, fix the HAM radio, the torn wiring in the attic, and fix the telephone in the library. His presence in the game, although optional like the rest of the kids, is significant (and perhaps canonical) because he reappears in Maniac Mansion: Day of the Tentacle as the main playable character.
(Hey Chris, you should probably change all of this. – mrfb)