In short, Esmeralda embodies freedom, empathy, and non-conformity. She is a nomadic outsider who is considered an illegal and beneath even the common poor. Lowest of the low. And, yet, she shows only kindness and compassion to Quasimodo and others who are wronged. Perhaps it is because she knows this feeling of being an outsider all too well. She rebels against the law of the land in exchange for her own moral code. She puts people above the law, something Frollo does not do. She is everything that Quasimodo wants in life: freedom, adventure, kindness, independent etc. This is probably why he falls in love with her immediately. Esmeralda, although kind and compassionate, does have a bit of a distaste and short-temper when it comes to the upper-class nobility. She is extremely untrusting and hesitant to open herself up to other people because she has been cheated before. This is why she is extremely skeptical of Phoebus's intentions towards her when he begins to show a romantic interest. It isn't until she sees Phoebus's ability to change for the better that she too opens herself up to change her opinion of him and others. Living life on the run could definitely cause some trust issues and a need for distance. Although she is kind and empathetic, she certainly lacks a certain connection because she has had to close herself off in order to protect herself. However, in the end, despite witnessing first hand some of the horrors that others can commit, she still takes an idealistic and empathetic perspective and Quasimodo's and Phoebus's capacity for change inspires her to open her heart to them both. Although she falls in love with Phoebus romantically, she certainly falls in love with Quasimodo in a more platonic way. In the end, she increases her capacity for love by fulfilling her want for a trustworthy companion/partner.
As for Phoebus, he actually starts out as the complete opposite. He is Captain of the law of the land. He represents lawfulness, safety, and self-absorbed ignorance. Phoebus starts out more like an anti-hero. He fights and works only to fulfill his personal wants and desires, he does not look past his own needs. He doesn't really have a strong moral code like Esmeralda. It isn't until he falls in love with Esmeralda that he begins to open himself up to the harsh truth that he is perpetuating a society built on unjust hierarchies and discrimination. Once Esmeralda is put into harm's way, he begins to understand personally the kind of loss that many of the lower-class citizens have been experiencing. Phoebus starts off extremely traditional, like Frollo, but what differentiates him from Frollo is his willingness to change and admit his wrongs. Frollo has a clear set of morals and goals and he openly discriminates. Phoebus, on the other hand, is simply guilty of being a man of his time. It was not until he experienced true love that he actually began to understand how weak he was for standing idly by while millions of people are in disarray. However, by the end of his character arc, he symbolizes something nobler: redemption, capacity for change within a close-minded culture, and justice. From only servicing himself to becoming a servant to others in need, Phoebus goes from using his title as Captain to benefit himself, to truly earning that title by becoming a defender of the people.