They are childhood friends. They don't have a romantic past. The following is a spoilers.
Liu's ex-fwb uses Shushu for money and fame. They were dating and Shushu's feelings for ex-fwb were sincere. That's why Liu didn't tell his past with his ex-fwb to Shushu, thinking maybe their relationship will be really something long-term. But in the end, things ended badly between shushu and ex-fwb.

wait can someone pls explain to me the dynamic that the ML and Shushu share? i wanna know why the ML was so protective of him and if they share (or shared) some sort of romantic connection or smth (i’ll lowkey get bothered if it’s some sort of love triangle situation huhu )