is a junior
. Anyone you go or went to school with or work with or used to work with who started after you is a kouhai
. Also, anyone who is younger than or less progressed in something than you.
The best example I can give you is that everyone in E2 that is of a lower level than you is your kouhai. Everyone that is higher than you is a senpai.
I always try to look after my kouhai from school and work. I try to keep in touch as much as possible and go drinking with them very often. I teach them about hashigo, nanpa, and goukon and the way of the salaryman because as their senior, it is important in their growth. I try to see them as much as possible even though some of them live far away in places like Nagoya, Osaka, Aomori, and Fukuoka.
kouhai is written in kanji and is a noun.