What is the word Junior when translated from English to Japanese?

The Word for the junior is kōhai (後輩) he is considered a junior of his Senpai (先輩)

Senpai is often seen romanized as "sempai" because it is pronounced that way (the Japanese "n" (ん) is pronounced as "m" when it comes before bilabials, such as "p").

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.