[1] Kōyō played ball for the Terao Dolphins as a fifth-grader in primary school, and went on to pitch for Namamugi Junior High.

He became the ace pitcher of Kawasaki Kikoka High School in Kanagawa, but his team never made it to any national tournaments.

While Aoyagi joined the main squad during spring camp and got a shot at pitching during one preseason game against the Chiba Lotte Marines, he struggled with control as he walked the first 3 batters he faced, and ended up giving up 2 runs in 2 innings of relief.

[11] Aoyagi's next start was on July 7 in the Tokyo Dome, where he shut down the Yomiuri Giants by pitching 7 perfect innings while allowing only a single hit.

Despite a strong start, Aoyagi lost momentum during mid-season, and he finished the season with only 4 wins out of 13 games with a 3.29 ERA.

During the post-season break, Aoyagi pitched for Samurai Japan (national team) in the Asia Winter League tournament in Taiwan.

[13] Aoyagi's good performance during spring training and preseason exhibition games earned him a spot in the roster at the start of the season.

[14] As Aoyagi struggled with control in his next two starts and committed errors that lead to runs, he lost both games and got sent back to the farm at the end of April.

[15] He got called back to the main squad a few weeks after, and notched his first win against the Yokohama DeNA Baystars on May 28 (6 innings, 8 hits, 2 runs).

Aoyagi was eventually called back to the main squad, and was assigned to start the September 2 match against the Baystars, where he recorded his first win of the season.

[22] But as the season progressed, Aoyagi racked up more losses than wins when he hit a slump in August & September (6.41 ERA).

Aoyagi won all of his four starts in June and notched a personal best of 1.20 ERA which earned him his first ever Central League MVP of the month award.

[27] He continued his winning streak, and by midseason, topped the league in ERA (1.79) and quality start percentage (85.7%), both of which helped him get voted into the All–Star game.

[34] His fielding skills also leave much to be desired and is a work in progress as he has trouble making accurate throws during assists.