Complex sentence: Nicknamed "Triton" by Mana, Gai and the Ouma siblings became close friends and spent a summer together where he admired Shu for his confidence and fell in love with Mana for saving him.
Split into simpler sentences: Nicknamed "Triton" by Mana, Gai and the Ouma siblings became close friends and spent a summer together. During this time, he admired Shu for his confidence, and fell in love with Mana for saving him.