He acts like the older brother of the camp. Because he wants to raise the level of all the other courts, even though he's really a 1st Stringer, he kept himself on the 5th Court.

Oni is seen in the U-17 camp in the 5th Court, sharing in common with Irie Kanata the fact that they are in lower courts than than their actual skill level. Despite being in 5th Court, the members of the 1st Court comment on how powerful he and Irie are who is in the 3rd Court. Afterwards, during the shuffle matches in the U-17 camp, the 5th Court defeats the 3rd Court due to Irie pretending his arm hurts and not being able to play meaning the 5th Court replace the 3rd Court making Oni a member of the 3rd Court.

(Source: Prince of Tennis Wikia)