Senpai and kōhai
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In Japan, senpai (先輩?) (also sempai) is an upperclassman, someone of a higher age, or senior and kōhai (後輩?) is a protégé or junior. The mentor system is found at all levels of education, and in sports clubs, businesses, and informal or social or ationship is an essential element of Japanese seniority-based status relationships, similar to the way that family and other relationships are decided based on age, in which even twins may be divided into elder and younger siblings.
The senpai is roughly equivalent to the Western concept of a mentor, while kōhai is roughly equivalent to protégé, though they do not imply as strong a relationship as these words mean in the West. More simply, these may be translated as senior and junior, or as an elder compared with someone younger in the family, company, or organization; the terms are used more widely than a true mentor/protégé in the West.
In Japanese school sports clubs, such as baseball teams, the kōhai are usually expected to perform various menial tasks for the senpai including washing clothes and cleaning. The kōhai may not be allowed to play the sport at all or have only limited opportunities to do so until they become senpai.
More than simple seniority, senpai implies a relationship with reciprocal obligations, somewhat similar to a mentoring relationship. A kōhai is expected to respect and obey their senpai, and the senpai in turn must guide, protect, and teach their kōhai as best they can. Senpai / kōhai relationships generally last for as long as the two people concerned stay in contact, even if the original context in which the senpai was senior is no longer relevant.