Complex sentence: She always hated the name until one day a new boy starts in the same class as her called Yūnosuke Ogasawara, the same day she is teased by a boy called Ken-chan about her nickname.
Split into simpler sentences: She always hated the name until one day a new boy starts in the same class as her called Yūnosuke Ogasawara. The same day he joins, she is teased by a boy called Ken-chan about her nickname.