Diamond no Ace Wiki
Yūki Masashi
Masashi yuuki
Seidou High School - No. 15
Left Fielder
Bats: Right Throws: Right
Kanji 結城 将司
Gender Male
Age 15 (debut)
Date of birth 3 August 1975
Place of birth Tokyo
Family Yuuki Tetsuya
(Older Brother)
Education 3rd Year Junior High (debut)

1st Year High School (current)

Style power batter
Skills Overwhelming batting power; body balance; good running ability
First Appearance
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. He is currently number 15 on the first string for the Seidou team in the summer tournament.


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.

Act I[]

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.[1]

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.

Act II[]

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.[2]

However, after his debut in the seidou first string, he was struck out numeorus times during practice games. He is rotated as an outfielder and has shown to have trouble fielding at times notably causing fielding errors. The third years Asou, Seki and Mimura are seen giving tips to the first years on fielding, making sure their catching and throwing abilities are up to par come the summer tournament. During the summer training camp he is shown to have a liking for shogi, and even asks Miyuki for a game as he states his brother told him that Miyuki was the best shogi player on the team. This takes place towards the end of the summer training camp, when the first string players are shown to be having a gathering in Sawamura's room, similar to that of the gathering in Miyuki's room with the previous third years the year before.

He is number 15 on the summer tournament team and one of the 3 first years to have made it on the roster. He proves to come in handy as a power hitter, as seen in their first game of the summer tournament where he ended the game in the 7th inning with a home run.

Relationships with other Characters[]

Yuuki Tetsuya []

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. They both share an interest in playing shogi.

Yui Kaoru[]

Masashi and Yui were the first two first years to join the first string before the Spring Tournament. They both are seen together whenever they observed the upperclassmen's practices. They also happen to be classmates. He ruined Yui's train of thought due to his frequent stomach growls.


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.[3]

Player Statistics[]



  • 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
  • He is more confident about his own ability than his older brother
  • He committed identity theft
  • He is wanted in 14 countries (ep 181)

Name Etymology[]

  • 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." [4]
  • "What crazy power!" - Spectators [5]



  1. Act I - Chapter 325
  2. Act II - Chapter 30
  3. Act II - Chapter 24
  4. Act II - Chapter 12
  5. Act II - Chapter 22