
Hangyum is the former friend. According to chapter 8 ans 9... Hangyum and Hajin did not date in the past. Hanwool and Hajin never meet (chapter 21). Flashbacks with Hangyum are high school (2 years). Flashbacks with Jaehee over a year ago, after high school. Not sure when they started to date or how long.

No problem and I understand........ but it is worst. Than that!!! Okay Twins I understand..... but yeah too many with Ha...
Hangyum, Hanwool, Hajin, Hauen. Han (Woojae). Han (Jaehee).
Take the last of Hanwool and first for Jaehee and you all get Woojae... -.-; And if you take Woojae Han. You almost get Jaehee Han
There are many Korean names, they could had different names

If I understood correctly the mc only dated this one ex that is back now (the one with the girlfriend); he also almost dated the white haired guy, that he's having a fling now, in high school. And probably never met the twin of the white haired guy but he thinks that he's the one that he met in high school . Sorry if it turned out confusing or if I wrote anything wrong ( ̄∇ ̄")
Er... I'm confused. Did the mc date the three of them at some point (currently, the other white hair mc and his twin and that dude with a girl in the past)? ._."