My favorite is definitely Ryou Fujibayashi, from Clannad, which is odd because she really doesn’t get enough development to really establish herself. Certainly not as much as Kyou does.
What I like about Ryou is that there’s more to her character than meets the eye, and her failings as a character seem believable and sympathetic. When we are first introduced to her, it’s established that she’s diligent, responsible, concerned with others (including the wayward Tomoya), but also pretty shy and a little clumsy. She also has this quirk with her whole fortune telling schtick. From there we’re introduced to Kyou, who basically barges in and steals the spotlight. Kyou has her own character music and a bold personality to match. From the moment Kyou steps in, this is Kyou’s story and Ryou is basically a tool for telling that story.
Spoiler: Kyou’s Clannad Arc What strikes me about Ryou is how her literary function and character motivation are so complementary. The fact that Ryou is a supporting character tacitly reinforces her motivation to outshine her sister and have Tomoya. She doesn’t /really/ want to be with Tomoya so much as she doesn’t want her sister to have him. She wants to be first in someone’s heart, something that I think we all desire, but she’s always in her sister’s shadow.
Ultimately, I like the character growth that I see in Ryou during her sister’s arc. She stops being selfish, and puts her sister’s desire for true love over her own desire for mere validation. Looking at the story from Ryou’s point of view, it’s kind of a bittersweet ending, but one that shows her strength as a character.
Ironically, I don’t think Ryou really shines in the Kappei arc. We still see her strength and resolve, there, but we don’t really see a lot of growth.