Who is jiri guys? What is his relationship with all of them?

The teacher. Note says he's called jiri cuz of the subject he teaches but not his real name

so in "for your love" teacher was almost always referred to as "jiri" which means geography (he was the geography teacher) the students still call him that, so when the little cutie student is talking about sunwoo he refers him to the name all the hyung-nims would have been familiar with (because all three of them went to that same high school)

ohh that teacher who is our main lead (๑•ㅂ•)و✧

yes !