Masao Nakayama is a Japanese defender in the Captain Tsubasa series. He joined Nankatsu SC during elementary school through selection trials, catching coach Shiroyama's attention. He established himself as a regular defender alongside Urabe, Kishida, Takasugi, and Ishizaki. He later transferred to Otomo Middle School with Urabe, enduring rigorous training to challenge Tsubasa Ozora. At Otomo, he formed the "Otomo Quartet" with Urabe, Kishida, and Nishio. The quartet ultimately lost to Nankatsu in a prefectural match. Following this defeat, he supported Nankatsu in the national tournament. Subsequently, the entire Otomo Quartet enrolled at Nankatsu High School. For Nankatsu High, he played in matches like the one against Toho Academy, wearing jersey number 3. He attended Japan's under-20 training camp as a candidate for the World Youth Asian Preliminaries preparations but did not feature in any official matches. His participation in Japan's professional J1 League is unconfirmed. In later arcs, he appeared as one of 33 local players invited to Japan's Olympic training camp for the Golden-23 and Rising Sun tournaments. He was among the first group cut from the Olympic squad. After Japan's Olympic quarter-final against Germany, he and other former Nankatsu High teammates aided the injury-depleted national team. His known techniques include the "Strong Tackle" or "Tremendous Tackle," a powerful sliding challenge to dispossess opponents. Cooperative tactics include the "Combi Play" with Urabe and the "Otomo Quartet" formation. Outside the manga, the Tecmo game series depicts him differently: he joined Nishigaoka Elementary School before moving to Nankatsu SC, contradicting the manga. Later Tecmo game appearances show him with Nankatsu High, Japan Youth (as a substitute), Seinan Miracle, and attending training camps without final selection. In international dubs, the character is named "Marcos" in Latin American versions and "Jill Taylor" in some European localizations. His current professional activities or club affiliations beyond high school remain unspecified in official media.