Ada – Jarndyce's ward, Richard's cousin and eventual wife – is sort of a non-character. She is all goodness, beauty, purity, and honor, which makes her kind of bland and not particularly interesting. If Esther is a somewhat complexly presented picture of a near-perfect woman, Ada is more of the same – minus the complexity. She is beautiful, good, and sacrificed to the good-for-nothing Richard and his obsession with Chancery court.
The only thing that makes Esther stand out from the cookie-cutter mold she was made in is her relationship with Esther. Their bond is hard to understand, but it seems crucial to Esther's personality. What do you make of the fact that when Esther is stressed about her facial scars, the person she is most anxious about seeing her is Ada? Why does Esther cry so hard about Ada's marriage to Richard, and why does she secretly come back at night to touch the door of their new room? Is there a way to connect Esther's love for Ada to Lady Dedlock's fondness for Rosa?