Complex sentence: While he makes it obvious that Kotoko causes him trouble, he admits that she has made his life more interesting, allowing him to experience feelings (such as excitement and nervousness) for the first time.
Split into simpler sentences: While he makes it clear that Kotoko causes him trouble, he admits that she has made his life more interesting. He admits how she has allowed him to finally experience human emotions (such as excitement and nervousness) for the first time.