Probably Hachi. I rarely like maincharacters, but Hachi is just so well-written and interesting. I like how realistic she is. She's likeable and cute, but constantly makes terrible decisions all the way to the end. She has a very interesting character development imo, but it's still very realistic. While she does change throughout the anime/manga, she isn't necessarily a better or a worse person at the end. I think that makes her a very interesting, but also heartbreakingly tragic character.