Kōsuke Izumi (泉 孝介 Izumi Kōsuke) is Nishiura High School's baseball team's center fielder, and currently the team's only switch hitter.
Izumi is one of the more experienced players on the team. He is good at batting, a fast base runner, and very good at catching. During the official games, he is shown to be a switch hitter, and can bat both right-handed and left-handed.
Izumi has almost shoulder-length brown hair and gray-ish blue eyes. He is more often seen wearing the baseball club's or school's uniforms. He also has some freckles on his cheeks.
Izumi is known to be both fun-loving and a bit flippant. He is childish in his enthusiasm for games, such as jungle gym freeze tag that are played during practice, as well as his squabbles with Hamada. He is also observant of those around him and often is the one to reign in Tajima when he steps out of line. Izumi is also a hard worker, as he is shown practicing late at night during a montage near the end of the second season. He also notes that while he hated practice in middle school, he enjoys it now.
Izumi attended the same elementary and junior high school as Hamada, although up until high school Hamada was a year above him. He appears to enjoy antagonizing Hamada and does not treat him with the respect due to a former upperclassman.
Izumi is in the same class as Mihashi and Tajima. He shares their lunchtime activities of talking, eating and then falling asleep. He is also supportive of Mihashi, and once thought of "doing it for him" while diving to catch a difficult ball. He appears to be upset about the prospect of being left behind in their friendship when he finds out that the two are already calling each other by their first names.
Izumi shows exasperation toward Abe's antics regarding Mihashi on multiple occasions, and seems to think that Abe is overbearing when it comes to the pitcher. However, he acknowledges Abe as an experienced baseball player, following his calls and gameplans.
He also seems to enjoy antagonizing Mizutani in particular, especially the incident where Mizutani confused the first aid acronym "RICE" with the food of the same name during the game against Bijou. Izumi continues to tease Mizutani for the mixup by calling him by the nickname "Rice" long after the above-mentioned game had ended.
- Izumi made a cameo appearance in episode 1 of xxxHolic Kei. Izumi's voice actor also voices the lead in xxxHolic, Watanuki.
- He takes 20 minutes to go to school and 40 minutes to return from school to his house.
- Izumi's favorite onigiri flavor is salmon roe. This is shown in the manga when Shino'oka checks a chart for the winners of the games played during practice, and she notes which fillings they like best.
- His hairstyle has changed the most at the course of the series. In his debut, his hairstyle was similar to Abe's. In the game against Mihoshi, it became a bit longer. Finally, it settled down to his current appearance.