Most likely the old guy, though the love interest is a close second. Think about it, how common is it that the "old guy" is the mentor or elder to one of the characters in the game, who is later driven by the desire to avenge said "old guy"s death?
I really wish I still had my Genesis and Sonic 2. I loved that system to death, then for some reason the P2 slot wouldn't accept up on the D-pad no matter what controller I used...damn near broke my heart. Sonic doesn't feel the same without the Genesis controller.