Semper is a entity that will randomly appear around the map. And is a neutral entity. Though he can exit GMC13B and show up on other maps on rare occasion. Semper is one of the entities that predate GMC13B and was most likely trapped within the map.
After the AlyxDoll and the Chair are combined and destroyed Semper will teleport you around the map. Semper has a chance to steal the Curse detector if the player freezes it on his head using a physgun.
If you stand too close to Semper and wait, he will start looking at you and then dissapear
Semper has no sounds though this could change in the future.