Wendy doesn't see a lot of her father, Garrett, growing up, and it's no wonder: Dude's a total free spirit who doesn't like to be tied down by responsibilities or obligations. Even his girlfriend Carolyn admits that she can't expect too much from him—she'll just end up disappointed if she does:
The reason it works for us, Carolyn said, is because I don't expect too much of him. That's all well and good for me. We don't have kids. All I've got is a bunch of cactus plants. It would be different if you had a kid with a person like that. (20.102)
Garrett's free spirited nature is what leads him to disappoint plenty of people in his life, though. He disappoints Janet by cheating on her and refusing to settle down as a family after she has Wendy, and he definitely disappoints his mother, who just wants him to have a fancy white-collar career. Instead Garrett wants to be an artist when he's younger and goes into carpentry as an adult—refusing to finish college like his mother wants him to.
After Janet dies, Garrett finally makes a real effort to be a real father to Wendy. When he invites her to live with him in California, he tries to be there for her, asking questions about her life and even taking her on a birthday trip to see the sights. Wendy appreciates this effort, but she is also frustrated that he hasn't been there for her until now. When she rants about how he's never been there for her, he agrees with her and says that he wants to make things right:
I wish I'd been a better father all these years, he said.
She was silent for a moment.
You've been doing better lately.
I didn't know what it was going to be like, he said. All this time, I stayed away partly because I was just so sure I'd screw up.
The staying away was the screwing up, she told him. (31.118-122)
In the end, Garrett does start to make amends with Wendy, slowly building a real relationship with her. He is there for her when she needs him the most, and she learns to trust in him, slowly, but surely.