Joyce Lanyon is a rich and beautiful widow who's way younger than Martin Arrowsmith. But that doesn't stop Martin from smooching her while his wife Leora is dying of plague in a nearby village. Martin doesn't know that Leora is dying, so he's not a total creep… just mostly a creep.
Joyce is a fancy upper class girl like Martin's first fiancé, Madeline Fox. But, as the book tells us, "amid the pretentious splendor which Roger Lanyon [her dead husband] had accumulated, Joyce was not tedious" (31.1.4). In fact, the thing that first attracts Joyce to Martin is the fact that he's nothing like her rich friends.
When asked by a friend why she likes hanging around Martin so much, Joyce answers,
"I quite agree that Martin is too aggressive, thoroughly wicked, very selfish, rather a prig, absolutely a pedant, and his shirts are atrocious. And I rather think I shall marry him. I almost think I love him!" (37.2.7)
Or in other words, Joyce likes Martin because he's unconventional and completely devoted to his passion—science. Little does she know, though, that it's this passion that will eventually tear Martin away from her.