What does 後輩 Kohai mean?

A kohai is someone who arrived after you at the same school, club or company. It is the other half of senpai, and neither term means anything on its own.

The characters

後 (ko) means after or behind. 輩 (hai) means fellow or companion, the same character that ends senpai. The pair is built symmetrically and deliberately: 先輩 is the fellow who came before, 後輩 the fellow who came after.

The obligations go both ways

English coverage of this pairing tends to describe deference flowing upward and stops there. The arrangement is more mutual than that. A kohai defers, does the unglamorous jobs and asks rather than assumes. A senpai teaches, covers, and carries some responsibility for how the junior turns out.

Neither party chose it and neither can resign from it. Arrival order is a fact, so the relationship is assigned rather than formed, which is what distinguishes it from mentorship in the English sense.

Why you hear it less

Partly because it is used less in address. A senior is called senpai directly, often instead of their name; a junior is usually just called by name. The word describes the relationship more than it functions as a form of address.

And partly because of where stories sit. Fiction about this dynamic is nearly always written from below, looking up at someone admired or unattainable. The junior is the viewpoint, and a viewpoint character rarely needs a label for themselves.

What the pair is actually for

It settles a question that English leaves open. In a group of peers with no formal hierarchy, who defers to whom? English answers with age, or job title, or force of personality, and often not at all. Japanese answers with arrival order, which has the advantage of being unarguable.

Whether that is a better system is a fair question, and it is a different question from whether English has an equivalent. It does not, which is why both words survive the trip untranslated.