For Gryffindor, courage is the key trait. A half - fish character could show great courage in leaving their underwater home and adapting to the very different environment of Hogwarts. They might also be brave in standing up to any discrimination or prejudice they face because of their appearance. Their unique form could give them an advantage in some Gryffindor - like challenges, like facing dangerous creatures in the Forbidden Forest or underwater areas around Hogwarts.