Professional tennis player Nobu Kurokawa was hit in the head by an opponent's malicious ball while participating in a world competition at the age of 20, so he traveled through time and became the younger brother of Byodoin Phoenix.
"Oh, Dad, I am still cool and cool today."
Ps: The author is a primary school student, if you have the guts, just jump into the trap [the series must be completed in your lifetime]