Mr. Hoo is a frustrated inventor who ends up running a Chinese restaurant. He seems concerned with not appearing authentically Chinese enough, so he adds "Shin" to his name and marries a native Chinese woman (Sandy's research shows he might have picked her because of her family recipes). He gets stomach ulcers from stress and is overly competitive with the downstairs coffee shop.
From the superficial information we know about the characters at the beginning, it seems like Mr. Hoo would be a good candidate for the murderer: he has ample motive, since he sued Sam Westing over the invention of disposable paper diapers several years earlier:
(Now, there's a likely suspect, Otis Amber thought. Hoo, the inventor; Hoo, the angry man, the madman.) (16.24)
But, as other characters soon inform us, he's so kind hearted he can't even kill bugs, much less people. He also gets rewarded for the court case, in a way, because the paper insoles he invents at Sandy's suggestion become very successful.