This forty-five-year old shop owner is wealthy and has a huge crush on Victoria Forester. In fact, he asks her for her hand in marriage right before Tristran swears to fetch a falling star for Victoria and runs off to Faerie. When Tristran returns and tells them to get married with his blessing, "Mr. Monday grinned, though it made him look as though he had a toothache" (10.93). Maybe he shouldn't smile if they get a wedding portrait.
He does seem to dote on Victoria, though, which is a plus, and he certainly has connections and wealth to go around. As Victoria tells Yvaine, in order to marry pronto they'll need "'special licenses which could only be issued by archbishops, and how lucky she was that Robert knew the archbishop'" (10.123). It sure sounds like he's got a good hook-up. And speaking of hooking up, it seems like they couldn't wait for the wedding, since Yvaine can tell that Victoria is recently pregnant.