Nerissa is Portia's woman-in-waiting (read: her sidekick). At the beginning of the play, she acts as a sounding board to Portia. She listens to Portia complain about her life and the unfairness of the casket contest and tells her to suck it up and be glad her father was wise enough to plan for his daughter's future. This, of course, tells us that Nerissa is a very practical girl. Hmm. This must be why she agrees to marry Graziano if Bassanio can win Portia. Read all about it in our analysis of Graziano.