MAROON: Nah! He's a Toon. You can drop anything you want on his head, he'll shake it off. But break his heart, he goes to pieces just like you or me. Read that.
Most friends would stop standing beside you if you dropped things on their head multiple times. They'd definitely stop standing under you. But Toons are different. It's in their nature to be slapstick and to hurt themselves for comedy, so they'll still be loyal companions to people who beat them up. But heartbreak is another matter entirely.
JESSICA (singing): Why don't you do right, like some other men do?
This is a classic jazz/blues song from the 1930s, making it a perfect fit for a film noir. As the resident femme fatale, Jessica needs to find a man she can trust. The catch is that most men like Eddie aren't going to trust her because she is the resident femme fatale. What's a girl to do?
VALIANT: She's married to Roger Rabbit?
BETTY BOOP: Yeah. What a lucky girl.
It's a running joke throughout the movie: why would a babe like Jessica be married to a goofy rabbit? Although she'll reveal the answer later, one main reason she likes him is because he trusts her and he's loyal to her. Roger only has eyes for one woman, and it's his wife.
MARVIN: Come my dear Jessica. I'm over here. I've got everything you need, right here, on the bed.
JESSICA: Oh, not tonight Marvin. I have a headache.
MARVIN: But Jessica! You promised.
JESSICA: Oh...all right. But this time take off that hand buzzer.
Eddie doesn't trust Jessica after catching her in the act of playing patty cake with another man. Seeing this makes it hard for him to believe anything she is saying. But later on, we learn she did this to protect Roger. She was always loyal to him after all.
ROGER: Somebody must have made her do it.
Even when faced with photographic evidence of his wife's patty cake indiscretion, Roger still trusts her. Eddie and Maroon think he's being ridiculous, but he turns out to be right.
BABY HERMAN: Valiant. The rabbit didn't kill Acme. He's not a murderer. I should know. He's a dear friend of mine. I tell you Valiant, the whole thing stinks like yesterday's diapers. Look at this. The paper says Acme left no will. That's a load of succotash. Every Toon knows Acme had a will. He promised to leave Toontown to us Toons. That will is the reason he got bumped off!
Roger needs to find some better character witnesses. His two biggest supporters are a foul-mouthed baby and an adulterous (in Eddie's mind) nightclub singer. Their testimonies don't inspire Eddie to swear his loyalty to Roger.
ROGER: Ha! We Toons may act idiotic but we're not stupid. We demand justice. Why, the real meaning of the word probably hits you like a ton of bricks!
Unlike Doom, who betrays his own people—the Toons—for personal gain, Roger is loyal to the Toons. He puts his own life at risk in order to save Toontown.