The following is a list of characters that appear in the baseball manga series, Ace of Diamond.
Seidou High School Eijun Sawamura Position: Pitcher
is the main protagonist of the series. 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 the ace player of Seidou. 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's ace. Eijun's strong points are his infallible spirit, bunting, infield defense, and his idiosyncratic pitches. His weaknesses are control, outfield defense, and batting. He later had overcome many of them, becoming particularly skilled at slash bunts and pick-offs. He has a hotheaded and loudmouthed personality, often causing his teammates exasperation because of his stupidity, yet it also rallies them during games at the same time. He maintains a friendship and rivalry with Furuya, a fellow first-year pitcher. Because his middle school team was so weak, he is extremely level-headed in critical times. The main balls he throws are called the "numbers". It involves combining Eijun's unique throwing posture and improving ball control with various styles of hand grips and throwing methods, each style being given a number, hence the name. Eijun and Miyuki developed this ability through accidental discovery after Ochiai tried to teach him how to throw a change-up.
Satoru Furuya Position: Pitcher, Left Fielder
is Eijun's rival, Furuya is a talented right-handed pitcher with extremely strong fastballs and forkballs. His 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 earned his 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 gained prominence within the high school baseball world and is described as a monster rookie. After the third years retired, Furuya becomes the ace of Seidou. However, as the story progress, the flaws in his playing style became more and more obvious. He later loses the position of ace player. Furuya's strong points are his incredible pitching, outfield defense, and batting. Being from Hokkaido, he was initially unused to the summer heat in Tokyo which drains his stamina quickly but he has since become accustomed to Tokyo's weather. However, his real weakness is his tendency to overcompensate when under pressure which affects him both physically and mentally; it was this weakness that ultimate costs him the ace position as Coach Kataoka realize the burden of being the ace was not allowing Furuya to overcome his problem. This decision ultimately paid off as Furuya began to regain his footing not long after and reclaim his reputation as a monster rookie. Personality wise, Furuya is stoic and unsociable with a cool demeanor and often has an expressionless face, but his aura completely changes while pitching. He dislikes all other aspects of baseball besides pitching despite his talents. However, he is also humble despite his personality, once mentioning that he never thought himself better than Eijun. He respects Eijun to a certain point and often accompanies the latter 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 is not playing, or when he wants to ignore somebody.
Kazuya Miyuki Position: Catcher
is a brilliant second-year catcher with an easygoing yet twisted personality to draw out the full potential from the pitchers. He is the main catcher in Seidou High School, making smart and bold gameplays. When he was young, he was extremely blunt with his words, which makes him being harassed and beaten up by his baseball teammates in his junior high, resulting in him pushing others away from his personal life. It is also shown that he is not very much close to his parents. Miyuki has a great respect for Chris as a catcher, as he was on the team that Miyuki couldn't beat. After the third years retired, Miyuki was voted as the new captain. Apart from being emotionally closed off from everyone else, Miyuki is perceived as having a manipulative and nasty personality. However, he is very knowledgeable and does care for the well-being of his teammates, especially for Eijun and Furuya, and he is also well-liked by his team. He is one of the few players whose room is often flocked to by other members. He is also shown to be highly resilient despite his appearance, keeping quiet about the injury he suffered during the semi-final game against Seiko in the Fall Tournament so he could still play in the finals. Once this was discovered, however, he was not allowed to play at the Jingu tournament given the doctor's advice of three-week recovery period, during which all he did was to look after the team and specifically comment on the sense of unity of the pitchers which he finds sickeningly well to his taste. He came back completely recovered to the team for the Winter camp. Later on, Miyuki helped developed Eijun's "numbers" after they both accidentally discovered a way to exploit Eijun's unique throwing style while the latter was being taught how to throw a change-up.
Haruichi Kominato Position: Second Baseman
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 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, Haruichi is a very calm and quiet person, often acting as tsukkomi to Eijun's boke. When he was a child, he and Ryousuke were much closer as the older brother often protect his younger brother from bullies. Haruichi 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 was very small compared to the other boys, and it was noted that he looks a bit like a girl. He is easily embarrassed, especially when praised. His relationship with Ryousuke is slightly strained because they play the same position, but he still endlessly admires his older brother. Though reserved, he is highly ambitious and aims to take his brother's position. As the story progress, Haruichi begins to display some of Ryousuke's more negative traits, but other than that, remains more or less of himself.
Yoichi Kuramochi Position: Shortstop
is Eijun's roommate, who is best known for his laugh. Kuramochi enjoys replying Wakana's text and wrestling Eijun whenever they fool around. He is a shortstop who, together with Ryousuke Kominato, forms a strong pair of middle infielders. He is assigned as the first batter due to his ability to run quickly. He likes to play video games during his free time. In contrast to his mischievous personality, Kuramochi is shown to be able to read and understand others well, being one of the few people with the closest understanding of Miyuki's personality despite the catcher's cheeky attitude. After the third years retired, Kuramochi became one of the team's vice-captains alongside Maezono.
Current first years
Position: Catcher
A highly-skilled catcher who decided to join Seidou after being impressed by their pitchers. Due to the actions of his former coach back in junior high, he has lost any passion of playing baseball and developed a cold attitude as a result; watching Seidou's game caused him to try and see if he could enjoy baseball once again. Okumura immediately marked Miyuki as a rival to surpass after becoming roommates with him, and is rather blunt with his words. Okumura's relationship with Eijun started off bad, with Okumura initiating an argument with Eijun due to finding him annoying during their first interaction. Ironically, Eijun's pitching was one of the reasons why Okumura had joined Seidou team. Miyuki later resolved their differences by getting Okumura catch Eijun's throws, with Okumura shocked by the power and unpredictability of Eijun's pitches. He then promises not to catch for Eijun until he makes it to the first-string team, a promise he kept once he was promoted, forming a new battery with Eijun during one of Seidou's most crucial practice games while Miyuki was not around, proving himself to be just as a good catcher like Miyuki.
Position: Second Baseman
Koushuu's friend, who also decided to join Seidou. Always seen with a cocky smile on his face, Takuma is noted for his running speed and is currently in the second-string team.
Position: Left Fielder
The younger brother of Tetsuya Yuki who decided to join Seidou. Like his brother, Masashi is noted for his powerful hits that almost always ends in a home run. However, he has low accuracy, usually missing half of his swings. He and Yui were the first of their batch to be promoted into the first-string team.
Position: Catcher
A first-year student who joined Seidou, Yui is considered a baseball prodigy due to his skills but is also discriminated because of his short size. He sometimes helps to catch for Eijun, expressing interest in his "numbers". He and Masashi were the first of their batch to be promoted into the first-string team.
Position: Pitcher
A soft-spoken and timid first year who is currently roommates with Eijun and Kuramochi, Asada is noted for his unique curveball.
Position: Pitcher
Kanemaru and Tojo's friend and former junior high classmate, who is currently in the second-string team.
Position: Right Fielder One of the players currently in the second-string team, noted for being inconspicuous but definitely has the ability.
Position: Shortstop