When we encounter Portia we discover that she is a wealthy heiress on the cusp of inheriting her father's entire estate if she meets the terms of his will - that she should marry the suitor who chooses the right casket. She is constantly accompanied by Nerissa who is her lady-in-waiting and also her closest confidante.
The close personal attachment between the two women is clearly conveyed in the manner in which they communicate. Portia hardly sees Nerissa as a servant but deems her a friend. She divulges and shares all her thoughts, feelings, desires and frustrations with her. Their relationship is obviously founded on a deep trust and respect for each other. Furthermore, their bond is clearly the result of a long-standing association which had been established when both were much younger. The two appear to be of the same or a similar age and would thus understand each other better, easing their conversation since they would essentially be speaking 'the same language.'
Portia comfortably expresses her displeasure with the current line of suitors to Nerissa, who encourages her mistress to speak openly and freely. Nerissa obviously realises that Portia needs an outlet to vent her frustrations and she becomes her 'shoulder to cry on.' Added to this is also Portia's vexation about her father's will which she deems 'the will of a living daughter curbed by the will of a dead father.' As a caring and understanding friend, Nerissa avoids being too critical and advises Portia to rather see the good in her father's determination.
The depth of the two girls' relationship is further illustrated by the fact that Portia asks Nerissa to accompany her to Venice, in disguise, to act in Antony's defence against Shylock's malicious demand. This is further emphasised by the fact that Nerissa acts as her assistant during the trial and both girls encounter a similar complication with regard to the rings they had each given their husbands, with similar outcomes.
We learn of Bassanio's interest in Portia when he asks Antonio for financial assistance to woo the wealthy heiress. Bassanio wishes to stand an equal chance to win her hand but needs money to do so. He tells Antonio:
In Belmont is a lady richly left;
And she is fair, and, fairer than that word,
Of wondrous virtues: sometimes from her eyes
I did receive fair speechless messages:
Her name is Portia, nothing undervalued
To Cato's daughter, Brutus' Portia:
Nor is the wide world ignorant of her worth, ...
He is clearly infatuated with this lady 'of wondrous virtues.' He also makes it clear that Portia has shown interest in him since she had given him 'speechless messages' with her eyes. Portia must have given him loving looks which have convinced him that he stands a good chance of winning her hand.
When Bassanio arrives at Belmont, he is encouraged by Portia to choose the right casket. She makes it pertinently clear that she desires Bassanio and would stay his visit if she has to. She also mentions that she would have advised him how to choose if it had not meant the forfeiture of her inheritance. She intentionally wastes time in conversing with him so that she may enjoy his company for longer because he might just choose wrongly.
Bassanio expresses similar sentiments and states that he is tormented by the idea of losing his love if he should make the wrong choice. He expresses his love for Portia but wants to get done with the task of choosing a casket so that his torment may cease. he, fortunately, chooses the right chest and is soon married to his beloved. The wedding is a rushed ceremony because he has just been informed about the trouble Antonio is in.
Further evidence of Portia's love for him is found in the fact that not only does she offer to settle Antonio's debt many times over, so that he may be at peace, but also that she ventures to Venice, in disguise, to defend Antonio. In the end, the two are united back at Belmont to enjoy their nuptial pleasures and live a life of love and prosperity.