The kanji mirror of senpai
Senpai combines 先 (sen, "before, ahead") with 輩 (hai, "peer, generation"). Kōhai swaps the first character for 後 (go/kō, "after, behind"), giving the straightforward counterpart "one who came after." Jisho glosses 後輩 as "junior (at school, work, etc.); younger people." Digital Daijisen, via Kotobank, lists the same institutional sense: someone below you in age, standing, experience, or skill, and someone who entered the same school or workplace later.
The pair is relational, not a permanent rank in society at large. A person is kōhai only relative to a particular senpai in a particular group. Leave that club or company and the label does not automatically follow you everywhere.
Obligations run both ways
Coto Academy describes senpai–kōhai as reciprocal: the senpai guides and takes some responsibility for the kōhai, while the kōhai shows respect and follows club or workplace customs. That is closer to a built-in mentorship channel than to a single English word like "junior."
The senpai guide covers the senior side, English meme usage, and the December 2025 Oxford English Dictionary entry. Kōhai has not followed senpai into general English dictionaries on the same scale; when English speakers need the junior role, they usually explain the relationship in prose rather than borrowing kōhai.
Age versus entry order
A kōhai is not always younger by calendar age. Someone who joins a company or club later can be kōhai to an earlier hire even when the newcomer is older. The same rule applies on the senpai side and is spelled out in the senpai FAQ on this site.
Everyday Japanese gratitude and farewell vocabulary sits in a separate cluster on LingoGrove: arigatō for thanks and sayonara for a farewell whose literal sense is conditional ("if that's how it is"), not a dedicated "goodbye" morpheme.