Rei and Koga are both in the light music club and the same unit (UNDEAD). Rei often looks out for the members of the light music club and UNDEAD, even though he appears to be sleeping in his coffin all the time.
Rei often teases Koga by treating and referring to him as 'doggie,' which makes Koga irritated and annoyed as he insists he is a 'wolf'. Even though Koga acts as if he does not care about Rei, Rei still manages to get Koga to do his bidding.
Rei, as aforementioned, cares deeply about his unit members and the club members—partially because he is the leader of both UNDEAD and the light music club, but also because he wants the best for them. Koga is also grateful to everything he's done and cares about him, as shown in his occasional moments where he reveals his thoughts and feelings in the event stories.
On AO3, ReiKoga is the sixth most written ship within the Ensemble Stars! tag; Rei’s most written, and Koga’s most written.
- They make up half of the unit, UNDEAD, along with Otogari Adonis and Hakaze Kaoru.
- They are also half of the light music club, which consists of the pair and the Aoi twins, Aoi Hinata and Aoi Yuuta.
- Rei and Koga were in a unit named 'DEADMANZ' with Hasumi Keito when Koga was still a first year.
- In the Compensation Festival event, it was revealed that Koga copied Rei's way of talking during his first year because of his admiration for him.
- During Koga's first year, he adored Rei and thought he was 'the coolest in all the world'. In Compensation Festival, it is also revealed that after Rei changed into his current personality, Koga 'sulked' and changed how he called him, along with other things.
- In Koga's first year, Rei gave Koga his guitar, which he still treasures greatly.
- Compensation Festival (especially Epilogue 2) and Reminiscence＊The Crossroads of Each One are Very Important™.