Rhombus, near-equilateral triangle and immortal (yes, I know the name is all wrong for a triangle) who for various reasons can percieve the third dimension.
Albert Square (name stolen from Stewart's "Flatterland"), narrator of Flatland
Ellery Quin, five-sided detective and mathematician.
I could also use Professor Hex, one of the sample characters in the rules, if I change his stats a little, but I'd prefer someone new. Any suggestions? Preferably names that are funny...