Koichi is a changed man ever since he quit drinking.
I thought it was my friend and said hi, but it was a completely different person.
Koichi used to diss AKB48 pretty often, but now he's a crazed AKB48 fan. It’s as though he's a different person.
A separate person is someone who is different from who they were before. I saw you a year ago and you were someone else. Now you're a different person, in a good way! You're a changed man.
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.