|Seidou High School - No. 19|
|Bats: Right||Throws: Right|
|Age||15 (debut) |
|Place of birth||Tokyo|
|Family||Yuuki Tetsuya |
|Education||3rd Year Junior High (debut)
1st Year High School (current)
|Skills||Body balance; good running ability|
|Manga||Act I - Chapter 322|
|Anime||S2 - Episode 28|
|Voice Actor||Takeuchi Shunsuke|
|“||What crazy power!||”|
Yuuki Masashi is a student of Seidou High and was the popular fourth batter of Kokubun Senior in junior high. Like his brother, he also attended Akadou Junior High
Masashi appears tall and well-built, with broad shoulders. He has a close-shaved haircut and a serious face with a cut on his eyebrows.
Masashi is very serious about baseball and dreams to be in the Major League. He also has a very competitive side and would often emits a fiery aura when he fired up watching others play, especially against other first years players.
Masashi is first seen in The Resilient Second-String where he observes Seidou High's intra-squad game with Takashima Rei. During the match, he had come to a conclusion that Seidou lacks offense and needs a player who surpasses his brother Yuuki Tetsuya at batting whilst implying himself to be the one.
He came to watch Seidou High's finals match against Yakushi High during the Fall Tournament where he reveals to his fellow middle school students Okumura, Seto, and Akamatsu that he'll be enrolling to Seidou High.
He and Yui Kaoru were promoted to the First-String shortly after they entered Seidou. During the quarterfinals match of the Spring Tournament, Masashi made his batting debut at the bottom of the second inning. Despite his display of powerful swings, he struck out swinging. In the bottom of the eighth, however, he hit a two-run homerun that concludes the match in a walk off called game.
Relationships with other CharactersEdit
Masashi is Tetsuya's younger brother. Tetsu told Miyuki, that Masashi has a tendency to do things at his own pace, but his first year self can't hold a candle to him.
He was the fourth batter in his middle school. He's very similar to his older brother in the way that he would like to play every position and is a very explosive hitter. He managed to hit a homerun in his first official debut match, an achievements much like his older brother. However, despite his impressive batting power and swing speed, he has been struggling to make contact against live pitches thus far. Masashi also has a really good throwing arm, but still has a long way to go where chasing flies is concerned.
- Due to his house being close to the school, Masashi lives off campus at home with his family; he comes to school for morning practice and breakfast, but brings a bento from home for lunch.
- Yuuki (結城) - Forming Castle
- Masashi (将司) - Prefect Officer
- Prefect - a person appointed to a position of authority
- "I don't want to rob myself of any opportunities, so I hope to play every position there is." 
- "What crazy power!" - Spectators 
|Seidou High School|
|Miyuki Kazuya • Maezono Kenta • Kuramochi Yōichi • Kawakami Norifumi • Asou Takeru • Nakata Ataru • Ono Hiroshi • Higasa Shōji • Seki Naomichi • Yamaguchi Ken • Kijima Rei • Kawashima Kengo • Shirasu Kenjirō • Mimura Ryōta • Watanabe Hisashi|
|Sawamura Eijun• Furuya Satoru • Kominato Haruichi• Kanemaru Shinji • Tōjō Hideaki • Kaneda Tadahiro|
|Yui Kaoru• Yūki Masashi • Okumura Kōshū• Mogami Takeshi • Asada Hirofumi • Seto Takuma • Kuki Yōhei
|Former players||Yūki Tetsuya • Tanba Kōichirō• Kominato Ryōsuke• Masuko Tōru• Isashiki Jun • Miyauchi Keisuke • Sakai Ichirō • Kadota Masaaki • Kusunoki Fumiya • Takigawa Chris Yū • Azuma Kiyokuni
|Managers||Fujiwara Takako • Umemoto Sachiko• Natsukawa Yui• Yoshikawa Haruno
|Staff||Kataoka Tesshin • Ochiai Hiromitsu • Takashima Rei • Kazuyoshi Oota|