Regarding the note on Boku:
Adults will often refer to boys as 'boku', and boys will use this as a personal pronoun. If a young girl hangs around boys or brothers who use 'boku', then it's common for these girls to also use 'boku' as a personal pronoun until the end of elementary school. Past middle school, there's a sharp decrease in girls referring to themselves this way, usually the ones that persist in 'boku' have a personality that refuses to grow up and conform with societal expectations. With Yamato, her insisting on using 'boku' fits her immature personality that wants to become 'Oden'.