I’ll take care of you, no matter what happens because you’re my favorite underclassmen.
The comrade who's behind you is your junior.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The reading for 後 is the long こう reading, though. Just think of your junior being こういち.