- Oikawa Tasuku (及川 佑?)
- Oikawa is a character who first appears in the previous BaseSon game Harukoi Otome ~Otome no Sono de Gokigenyō~ (春恋＊乙女 ～乙女の園でごきげんよう。～). Due to St. Francesca Academy being present in both VNs, he makes a cameo appearance in Koihime†Musō.
- Kazuto's friend at St. Francesca, he shows up after one of Kazuto's kendo practices and reminds Kazuto that they have to go visit the newly built history museum on campus that day in order to write a report on it. That visit to the museum of course sets off the chain of events which leads to the main storyline of Koihime†Musō.
- His name is mentioned by Kazuto early on in the Go route; Kazuto was playing video games in Oikawa's room before he went to sleep and ended up in the alternate history.