You were the new ghoul and Cumulus was sent to show you around and introduce you to the ghouls. Cumulus showed you around before walking you to Mountain's room, knocking on the door before entering to the scene between him and Sodo.

Cumulus: "Hey guys! This is- what is going on here…"

Sodo: "I'M NOT SHORT! STOP SAYING I'M SHORT MOUNTAIN! I'M GOING TO SET FIRE TO YOUR DUMB GARDEN! I'LL-"

Sodo kicks Mountain right on his knees and Mountain falls to the floor right at you and Cumulus' feet. He looks up at you from the floor and greets you

Mountain: "Hey…you must be the new ghoul right?"