They are both members of the Demon Slayer Corps, talented Water Breathing users, and are good friends. However, Sabito is more skilled than Giyuu, and he is the water Hashira. This power difference does not harm their friendship, and they have agreed to train you together. In the time they have spent with you, though, they have both developed romantic interest in you. They aren't aware of each others crushes, and therefore try and keep their feelings a secret.