Eijun Sawamura (沢村 栄純,Sawamura Eijun?) is the main character. Eijun is a left-handed pitcher who has extremely erratic throwing motions. His body is naturally flexible with supple joints, allowing him to immediately change the pitch’s direction. Because Eijun has been playing baseball without proper instruction, his pitches normally change direction right before the batter hits the ball. However, this makes catching the ball difficult, as most catchers cannot predict where the ball will end up. He was initially unaware of this quirk in his throws. His knowledge about baseball is mediocre at best, but he strives to improve his skills and become Seidou's ace player.
Eijun comes from Nagano where he played baseball in his middle school. His team was below average, often making mistakes during critical moments and losing almost all matches. However, Eijun has an extremely strong spirit which, no matter how hopeless the situation got, inspired his teammates to continue playing their best. His childhood friend, Wakana, was the baseball team manager. Initially, after Eijun had been scouted by Seidou, he had planned to refuse because he wanted to keep playing with his friends in the same high school. However, his friends encouraged Eijun to go to Seidou despite their reluctant feelings because they felt as if they were holding Eijun back. Eijun eventually goes to Seidou and boldly declares that he will become the team ace.
Eijun’s strong points are his infallible spirit, bunting, infield defense, and his idiosyncratic pitches. His weaknesses are control, outfield defense, and batting. His personality is hotheaded and loudmouthed, often causing his teammates exasperation because of his stupidity. He has a rivalry/friendship with Furuya Satoru, a fellow first-year pitcher. Because his middle school team was so weak, he is extremely level-headed in critical times.
Satoru Furuya (降谷 暁,Furuya Satoru?) is Eijun’s rival, Furuya is a right-handed pitcher with extremely strong fastballs and forkballs. Furuya’s pitches are so strong that no catcher in Hokkaido was able to receive them. As a result, he became alienated from his middle-school team because he was far above them in ability. After hearing about the genius catcher, Miyuki Kazuya, he applied to go to Seidou high school to play baseball with him, and immediately took respect when Miyuki easily caught his pitch. During the game of first years versus second years, Furuya jumped from the second team to the first team after only one pitch. After playing his first official game against Yokohama, Furuya became a living legend within the high school baseball world and is often described as a monster rookie.
Furuya’s strong points are his incredible pitching, outfield defense, and batting. His weaknesses are stamina and infield defense. Also, Furuya’s pitches are so powerful that they cause damage to his fingertips and nails, so he cannot pitch for very long. He is stoic and unsociable with a cool demeanor and often has an expressionless face. However, his aura completely changes while pitching, once being described as "a wild wolf who wants to pitch some balls". He dislikes all other aspects of baseball besides pitching. However, he is surprisingly humble, once mentioning that he never thought himself better than Eijun. He respects Eijun to a certain point and often accompanies Eijun to do extra practice at night. Because he was an outcast in his middle school, he is happy about but unused to the Seidou upperclassmen’s friendly but bossy treatment towards him, being content when he was ordered to fetch them drinks and uncomfortable when they excessively praise him. He also likes taking advice. He gets bored easily, disliking it when he cannot play in a match, and often falls asleep when he isn’t playing, or when he wants to ignore somebody.
Kazuya Miyuki (御幸 一也,Miyuki Kazuya?) is a brilliant second-year catcher with an easy going personality and a sharp eye. He is the main catcher in Seidou High School because the coach and other people trust him with a huge responsibility by catching the ball that the pitcher throws as well as making the gameplay. Miyuki has a great respect for Chris as a catcher as the only team he was never able to beat during the middle school was led by Chris.
Haruichi Kominato (小湊 春市,Kominato Haruichi?) is a first-year second baseman of the team, Haruichi is a naturally talented, all-rounder baseball player. Despite his small stature and frail appearance, he has pinpoint precision while batting the ball, able to hit it as wide and as far as he pleases, and control where the ball lands. He also is extremely talented at defense, able to catch a ball in the air and throw it while still floating. He is often noted to have amazing baseball sense, able to decide in a split second where to hit the ball, what to watch for, and when to steal. His ability in baseball exceeds his older brother’s, Ryousuke Kominato, who is first-string player and also a second baseman. The only thing lacking is his stamina, which he works hard on improving.
Born in the Kanagawa Prefecture, he is a very calm and quiet person, often acting as tsukkomi to Sawamura Eijun’s boke. He has a habit of saying things bluntly and often praises either Furuya or Eijun, causing them to compete against each other frequently. He plays reserve in the first-string team and occasionally feels envious of Eijun and Furuya for being able to play in a match. He is very small compared to other boys and it has been noted that he looks a bit like a girl. He is easily embarrassed, especially when praised. His relationship with his brother is slightly strained because they play the same position, but he still admires him to no end. Though reserved, he is highly ambitious and aims to take his brother’s position. When he was a child, their relationship was closer, with Ryousuke often protecting him from bullies.
Shinji is Sawamura's classmate who gets along well with Hideaki. He initially looked down on Eijun, but was eventually impressed and admitted Eijun's abilities. He was tasked to watch over Eijun, Satoru and Haruichi by Chris, and eventually broke through the first team during fall of his first year, competing with Shōji Higasa for the starting third baseman position.
Hideaki is a former top 4 pitcher nationwide during his middle school days. However, stellar performances from pitchers Eijun and Satoru has made it hard for Hideaki to break into the first team. He has since entered the first team as a center fielder, but has said that he won't give up on the mound.
Kuramochi is the roommate of Sawamura who enjoys replying Wakana's text in Sawamura's place. He is shortstop who, together with Kominato Ryuosuke, forms a strong pair of middle infielders. He is assigned as the first batter due to his ability to run fast. He likes to play video games during his free time after the practice.
Tetsuya was unanimously voted by his teammate to be the captain of the Seido baseball team. His motto is to 'lead by performances' as suggested by the coach. He is also the cleanup batter of the team who is able to hit a home run. Although he is a rather quiet person, his strength seems to radiate when he is in the batter box. When he first entered Seido, his generation was considered as the weakest batch by the seniors. However, he trains hard and is able to inspire other teammates. He plays shogi during his free time although Miyuki says he is still weak at it.
Isashiki is a loud-mouthed person with a strong attitude. He wanted to be a pitcher when he first entered Seido baseball team, but was suggested to be a center fielder instead due to his strong shoulder but inability to throw a strike. He can throw a ball from the outfield to the home plate without bouncing the ball. He is also a strong batter who usually hits the ball to the right, although he admits he cannot hit better than Tetsuya.
Masuko is a roommate of Sawamura who is able to hit a home run of Sawamura's moving fastball by forcing his strength through his batting. He was demoted to the second team once due to the error he made during a game. He likes to eat pudding.
Chris is the former catcher of the Seido baseball team before he injured his shoulder from overworking. He was paired with Sawamura in the beginning in order to bring out his potential. He is eventually asked by the coach to become the scorekeeper of the team.
Kataoka is the coach of Seido baseball team. He rejects the offer from the pro baseball team in order to become the coach of his alma mater. He resigns after his team loses to Inashiro industries in the last round of Koshien qualifying tournament. At the same time he requests the Seido school board to let him remain the coach for the current year as his resignation may affect the morale of the baseball team and will also cause the new coach difficulties in training the team in the middle of the year.
Ochiai is the new assistant coach of Seido High school baseball team. He was recruited by the school after Kataoka hands his resignation. Ochiai will become the head coach of Seido high baseball team after the end of the school year.
Rei is the scout of Seido High school baseball team. She scouted Eijun Sawamura after seeing his middle school team's match which they lost. She believes Sawamura has the potential to become the ace of the team.
Narumiya is considered to be the best pitcher in Kanto due to his ability to throw strong fastballs, horizontal and vertical breaking balls, as well as the changeup. He was the teammate of Miyuki during middle school. Narumiya joined Inashiro along with the other strong baseball players in order to form the best team in his generation. However, Miyuki rejected his offer and went to Seido instead.
During the Koshien championship Narumiya is nicknamed, 'The Prince of the Capital' by the mass media for his exemplary performance.
Carlos Toshiki Kamiya (神谷 カルロス 俊樹,Kamiya Karurosu Toshiki?)
Yeung Shunshin is a second year high school student of Akikawa Academy, who comes from Taiwan. He is a pitcher of the Akikawa Academy baseball team. He came to Tokyo due to a language exchange program, but he got fascinated with Japanese baseball. He was nicknamed "Precise Machine" because of his incredible control
Ace of Diamond was the 25th best selling manga in 2011, with 1,711,607 copies sold.Nikkei Entertainment magazine published a list of top 50 manga creators by sales since January 2010, in its September 2011 issue; Yuji Terajima, the author of Ace of Diamond was ranked 20th, with 2,792,000 copies sold. It was the 27th best selling manga in 2012, with 1,685,194 copies sold. In 2013 Ace of Diamond became the 23rd best selling manga, with 2,010,045 copies sold.