The Baby (unofficial name) is a non-hostile monster in P.T..
The Baby is a small red creature that appears in the bathroom sink in the never-ending hallway. The baby itself seems to be a bloody, very young human fetus writhing in agony and suffering. Its heart appears to be outside of its chest, similar to ectopia cordis. It lacks ears and a nose; its eyes are barely visible and never open (although the eyelids slightly flutter); and its limbs are short stubs, emphasizing its weakness and futility, unable to crawl out of the sink.
Its only behavior is to wriggle around in a bathroom sink and cry. At some point in gameplay, a crying or giggle emits out in the hallway; it's not beyond reason that this is emanating from the Baby. The player can also trigger an event that makes the Baby a vessel for actual speech.
The hallway in which the player traverses is haunted by a ghost named Lisa. Various demonic voices, along with multiple text pieces, radio dialog, and other clues, reveals that Lisa was shot in the belly by her husband while she was pregnant. It is highly likely that the Baby is a representation, if not an outright spectral creature, of the dead baby.
Credence is lent to this theory by Lisa's occasional presence in the bathroom. She will often appear in the mirror, where she approaches the sink containing the baby. Near the beginning of the demo, Lisa can be found in the bathroom, where she will shut the door and lock the player out when the door is approached. Eventually in one visit to the bathroom, the baby will begin to speak with the voice heard on the radio, talking about how the player character lost his job and his wife had to get a job in a grocery store, which she would not have even gotten if the manager hadn't "liked how she looked in a skirt". It's implied, through this, that the baby is the manager's, through an affair with Lisa. The Baby implies that the affair happened approximately 10 months prior to the events of P.T.
- "You got fired, so you drown your sorrows in booze. She had to get a part-time job working a grocery store cash register. Only reason she could earn a wage at all, is the manager liked the way she looked in a skirt. You remember, right? Exactly 10 months back."
- "Dad was such a drag. Every day he'd eat the same kind of food, dress the same, sit in front of the same kind of game. Yeah, he was just that kind of guy, but then one day he goes and kills us all! He couldn't even be original about the way he did it. I'm not complaining, I was dying of boredom, anyway. But guess what? I will be coming back, and I'm bringing my new toys with me."
- "Watch out. The gap in the door, it's a separate reality. The only me is me. Are you sure the only you is you?"
- The baby's appearance seems to very strongly reference David Lynch's Eraserhead, an inspiration of the series, which featured a similarly pitiful infant and industrial nightmare world.
- The radio, in one occasion, may broadcast a rather ominous message: "You can't trust the tap water." Sinks are often called tap, so it could be possible that the radio is warning the player not to trust what the Baby in the sink speaks later on.
- It can be speculated that the speech at the end of the teaser is told by the baby, as it mentions "Dad" and "kills us all".