That thimble she bought at the garage sale, unbeknownst to her, is a classic Ming piece, and the evil antiques dealer wants it real bad. Or the elderly neighbor is so grateful she leaves him her stock in the family corporation, and he blunders into a morass of corporate corruption and family dysfunction. In fact, it might produce admirable qualities. That translates into a slightly weird form of the admirable quality of independence. Also look to give this character some compensating strengths.
Wimps often have to develop a razor-sharp sense of humor to serve as a weapon and a shield.
Wimps might also be extremely good at arcane skills that come into play during the plot. They also tend to be loyal friends and persistent workers. I did think of Sandra Bullock as a nerdy computer programmer in The Net, but, well, she still looked like Sandra Bullock. He's just about the biggest nerd in the world, and yet his essential sweetness makes him loveable. Indiana Jones is scared of snakes.