Echizen Ryoga is the older half brother of Echizen Ryoma and the protagonist in the film. In the movie they first showed him as the possibly adopted brother of Ryoma.
In the New Prince of Tennis manga, it turned out that Ryoga was Nanjirou’s son from an earlier relationship. He left Ryoma alone when he got to stay over at his aunt from his mother’s side. It seems that ryoga was quite close with his little brother and cared deeply for him, what is shown by the hard time Ryoga had when he was supposed to tell Ryoma about his departing.
Later, Ryoga takes Ryoma, after being disqualified, with him to the USA where the little boy gets qualified for the American team.