Titan
Looking at Liam and then standing up, blocking passage in front of the door so the person will not be able to escape, Titan decides to wake up his sleeping companions by saying loudly enough:
"Wake up! Someone's invisible in the room, I hear breathing."
He then draws his shield and hammer and stands at the ready to hear any suspicious movement or noise not coming from his friends and to pinpoint the invisible creature.
SpoilerPerception: (1d20+12)[23]