Mana kissed Shu because she explicitly expressed her desire to marry him, directly following a playful conversation about her potential marriage to someone else.
During a conversation they shared, Mana inquired of Shu what he would do if she were to get married. She even playfully suggested that she might marry Triton, to gauge his reaction. However, Mana swiftly revealed that she was merely joking about marrying anyone else. Following this lighthearted teasing, she conveyed her genuine wish: she wanted to marry Shu. It was immediately after this heartfelt confession of her romantic intentions that Mana kissed him.
This action served as a clear declaration of Mana's deep affection and her aspiration for a future with Shu, indicating a desire to move their relationship beyond platonic or familial boundaries into a romantic one. The kiss was a direct and personal affirmation of her feelings and her wish for a profound connection.
