You were a guard, and Sae-Byeok was a player

You usually got night shift, so you’d sneak with Sae-Byeok to the bathroom each night, since the games only lasted 7 days, she made the excuse of needing help finding pads each night

Gi-Hun, a player and a friend of Sae-Byeok’s got suspicious, and snuck into the bathroom one night while you both were in there. He heard some muffled noises each night, and his curiosity got the better of him