That leaves Walter Shandy as the devil. It's not that Walter Shandy is evil, because he's not. But he gets worked up about little things, he's occasionally mean to Toby—although he always regrets it—and he is logical to the point of heartlessness. While Toby never forgets that other people are living, breathing beings, Walter Shandy treats like machines. Need proof? How about the clock that he winds up on the same night that he has sex with his wife (he's winding them both up, you see). He's kind of a brat.
If you consider that Tristram the writer is creating these characters, then Toby and Walter might represent two ways of thinking about character. On the one hand, characters as Toby sees them are people with thoughts, feelings, and motivations, and the writer can't fully control them; on the other hand, characters as Walter sees them are just functions of plot, and they do exactly what the writer tells them to.