Yu is a twin with Ai and Rin's younger sibling. Yu is a boy. They (Yu and Ai) are so naughty that they always trouble Rin.
Yu appears along with his sister, Ai, as they play and try to annoy Rin, they also steal Nozomi's cookie. Rin becomes angry and says them to stop. Seeing them sad, Rin gives them the ball, Yu and Ai decide to play with Nozomi and Urara, however they spot Coco, and Nuts and take them. While playing, Nuts become angry and speaks making Yu suprised, however Urara kicks the ball making Yu forgets about it. While playing Yu kicks the ball, and the ball falls at Gamao's hands, Nozomi and Urara with Yu and Ai, come and Nozomi than finds out and says to give the ball. Gamao seeing it tries to pop it, however Yu and Ai say to give the ball. Gamao getting annoyed throws the ball at them, Yu and Ai, get scared, however Rin comes and saves them, seeing their sister get hurt, Yu and Ai hug her. After Gamao's defeat, everything gets normal, as Yu and Ai hang out with Rin.
(Source: Pretty Cure Wiki)