Agnes Wickfield is an angel. We know it because David tells us so, like, a million times. But she also shows it through her appearance and behavior. We first encounter Agnes when David moves to Canterbury to go to Doctor Strong's school. David winds up living with Miss Betsey's business partner, Mr. Wickfield. The person who keeps Mr. Wickfield's house is his young daughter Agnes, who is so patient and wise that everywhere she goes, she seems to shoot out rays of calm and contentment. She's just – perfect. And because David is a fool, he notices that she is perfect but doesn't think to marry her until the end of the novel.
Anyway, so Agnes is basically too good to be true. She is completely self-sacrificing. Her father, Mr. Wickfield, lost his wife when Agnes was a baby. He relies on Agnes completely. And Agnes, far from resenting her father's obsession with her, is willing to stay patiently in his house supporting him as best she can. Even though Agnes recognizes that her father's fixation on her is unhealthy, she is so caring that she is more concerned for him than she is for herself, always living shut up in her father's dark house:
I almost feel as if I had been papa's enemy, instead of his loving child. For I know how he has altered, in his devotion to me. I know how he has narrowed the circle of his sympathies and duties, in the concentration of his whole mind upon me. (25.53)
This total devotion to Mr. Wickfield leaves Agnes vulnerable. As Mr. Wickfield's drinking gets worse and worse and he leaves more of his business arrangements to the fiendish Uriah Heep, Agnes starts to realize that Uriah Heep wants to force Agnes to marry him. And again, instead of standing up to Uriah Heep or her father, she begs David not to interfere:
While I have an opportunity, let me earnestly entreat you, [David], to be friendly to Uriah. Don't repel him. Don't resent (as I think you have a general disposition to do) what may be uncongenial to you in him. He may not deserve it, for we know no certain ill of him. In any case, think first of papa and me! (25.56)
Rather than risking any loss of Mr. Wickfield's reputation, Agnes is so self-sacrificing that she is willing to allow Uriah Heep to keep conspiring to get her to marry him against her will.
And Agnes's self-sacrifice gets even crazier. Even though she has clearly been carrying the torch for David forever (she tells David at the end of the book that she has loved him "all my life!" [62.71]), Agnes sits and listens as David tells her about all of his crushes on other ladies. David really treats her as a sister for the first five-sixths of the novel, detailing his feelings about Miss Larkins, Miss Dartle (well, he finds her interesting, but not lovable), and, of course, Dora Spenlow. Indeed, Agnes even comes to David's house to nurse Dora when she is dying! And Agnes sits and takes all of this from David, because what she's learned about love from her father is that it means complete, total self-sacrifice and self-denial. (For more on this, see our "Character Analysis" of Mr. Wickfield.)
Indeed, when Uriah Heep is finally booted out of Mr. Wickfield's business, Agnes takes it upon herself to start a school for girls so that she can support Mr. Wickfield in his old age. Agnes literally never does anything for herself.
After three years abroad trying to recover from Dora's death, David has come to realize two things: one, that Agnes loves him, and two, that he loves her. But David assumes that nothing can come of this love because he has rejected Agnes so thoroughly, for so long. When he gets back to England, Miss Betsey drops hints that Agnes has a fella. David, being stupid, doesn't pick up that Miss Betsey means David himself.
So, David goes to ask Agnes if she actually does have a guy – if she does, she should feel free to tell David, as he has always felt free to tell her about his little romances. Finally, David catches the clue that Agnes is in love with him. So, they agree to marry. (In the run-up to all of this, he keeps calling her "sister" (62.47), which gives it a whole new edge of weird.)
And the reason David says he loves her is because, "You will always be my solace and resource, as you have always been" (60.91). In other words, David loves Agnes because she is so self-sacrificing. He wants to keep using her emotional resources to make him a better man. David is relying on Agnes to "[point him] upward!" (64.34) for the rest of their lives.
Agnes is a model of the nineteenth century ideal of the "angel in the home," the woman who keeps her husband's house peaceful and calm while he goes out into the world to carve out a name for himself. Her support is what enables David to get the happy family life he has always wanted.
Poor Dora gets so much criticism during her life and after her death for not being able to keep David's house. Agnes, by contrast, is so good at keeping house that she supports Mr. Wickfield and David without them being fully aware of how much she is doing for them. Her entire purpose in life seems to be to smooth the way for other people – Dora, Emily (whom she visits often after Emily rejoins Mr. Peggotty, David discovers later), her students, Mrs. Annie Strong, Mr. Wickfield, and David. For more on Agnes as an ideal of womanhood, check out our theme discussion on "Gender" and our thoughts on "Character Roles."