In William Shakespeare's The Merchant of Venice, Nerissa is the intelligent and savvy waiting woman of the play's heroine, Portia, while Gratiano is the loud-mouthed but good friend of Bassanio. These supporting characters seem to do exactly that: support and serve as reflections of their protagonist counterparts.
The relationship between Nerissa and Gratiano develops as a mirroring of the relationship between Portia and Bassanio; when Portia and Bassanio get married after Bassanio passes Portia's father's test, Nerissa follows suit and marries Gratiano. This seems to make the bond between Portia and Bassanio even stronger, as it is a match reinforced by the union of the two people who are around them the most. Although neither Nerissa nor Gratiano are particularly independent or capable of resolving problems on their own, together they are able to join forces in reinforcing the integrity of their friends' relationship. Ultimately, they are loyal to each other and loyal to Portia and Bassanio.
Like Portia and Bassanio, Nerissa and Gratiano are in love and get married during the play. The eNotes study guide describes the two marriages as "almost a mirror image":
Nerissa imitates the actions and embraces the values of her mistress. In the copycat wedding of Nerissa and Gratiano and in the parallels of the ring subplot, The Merchant of Venice offers a lesson in Elizabethan social conduct: lower-class persons should mimic their social superiors.
The scene with the two couples and the missing rings in Act V serves as comic relief after Shylock's "trial."
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