He’s been friends with a boy named Matsuoka since elementary school, and they both took the same tennis classes. Back then, Matsuoka was the most talented, but by the time they entered High School, their positions reversed as Nishikori became the ace of the tennis club and was popular with girls, which caused Matsuoka to feel jealous and start avoiding his friend. However one day, they scheduled to meet for a morning practice, but on that morning he was hospitalized with a severe case of food poisoning and so could not come. He tried to contact Matsuoka as soon as he became better in the evening, but he could not reach him for he died. Since then, he’s been feeling a hand on his right shoulder, which bothers him while playing tennis, and any pictures of him with a girl would feature a ghostly hand in front. Suspecting his dead friend to be behind all of it, he sent a request to Rinne.
The spirit of Matsuoka is revealed to be indeed lingering, as Rinne and Sakura see him, and his actions are revealed not to be for jealousy but actually because he was taught by Matsugo, whom he calls him “Friendship Master”, many methods of restoring their friendship. Although all it took was for Matsuoka to be visible to Nishikori so they can both apologize to each other, since Matsuoka also did not show up for their morning practice, and thus his friend gets to pass on.