How can a past lovers meet in a fight?

A past lovers who both had a memory loss and lived their lives in secrecy and lies? 

Lee Chae Rin

Baddest female in Undertown. Toughest gizibe you'll ever met. A leader of a famous chic gang named SH. Short for Seductive Hypnotizers. The name itself only shows on what they can do and it's no joke tho.

But this girl was lived for 5yrs with full of lies. What she would do if ever she'll met a guy who had been a great part to her life?

Kwon Ji Young

Guy replica of CL they said. And that's undoubtedly right. Leader of a hot-mean-y gang whose named under MIB or Men In Black since they all love black. Simple. 

But how come this guy didn't know anything on his past? Can maybe CL has something to do with it? Or she's just a simple rival?