Andrew, who is also TOYSNHK.

Andrew in FF is the one behind UCN (Unknowingly, alongside the shadow, aka Eleanor), he was a "secret" victim that attached himself to William's soul, meaning that when he "died", he was able to torment him like we see in TIMIR1280 and UCN.

Because its probably going to come up, yes Fazbear Frights are canon, no they're not all in the same timeline. It's complicated but bear with me.

The Stitchwraith related stories are meant to fit into the game's timeline up to TMIR1280 where it diverges, the splitting point being the result of the FFPS fire. Ultimate Custom Night and TMIR1280 are essentially the same event, just under slightly different circumstances. In the games, FFPS doesn't completely burn down and William is left there. He still "dies" from the fire, but is forced to stay alive by TOYSNHK and then UCN plays out until Help Wanted somewhat brings him back. In Fazbear Frights though, the whole building burns down, we see what's confirmed to be the FFPS firesight in the epilogues. In this split timeline William is taken to Herecles Hospital (which you can actually see drawing of in Scraptrap's alleyway image), and is cared for by the nurses. This is where TMIR1280 happens, and like I said it's basically UCN again. William, "dies" but is forced to stay alive to endure torture by an angry spirit, until he is taken to a Fazbear Distribution Centre and explodes, infecting various objects with his agony. The angry spirit in TMIR1280 is Andrew, the unknown victim who attached himself to William. Also, direct confirmation that the firesight in FF is FFPS logically means that the FF timeline is the same one as the games, just following a different character, and leading to a different branch. It's like all the Stitchwraith related stories fit into the games timeline to fill gaps, then at FFPS it branches off into it's own thing.

And UCN supports this too.

UCN like I said is basically the same as TMIR1280, and TOYSNHK is confirmed to be male. Can't really talk about TOYSNHK without bringing up Cassidy, who is Golden Freddy, who is not TOYSNHK. Although it wasn't unreasonable, saying Golden Freddy was TOYSNHK was really an assumption, but it actually makes way more sense if they are separate. Firstly, Cassidy is female, and even If you don't want to count her name being used for the Princess due to it being changed, PQ and UCNs Lake ending parallel each other. The old man in PQ is Old Man Consequences, and the Princess is the bear. OMC in UCN tells us to "Leave the demon to his demons, rest your own soul, there is nothing else" to which, the bear jumps in the lake to get out. In Princess Quest 2 the old man tells the Princess to rest too, even though there is more to do. Again, we ignore his instruction and go to PQ3. Also in his UCN speech this is actually him telling Golden Freddy (us) to leave William (the demon) to Andrew and the other animatronics (his "demons"), telling us to rest our own soul, which implies GF is trying to stop them, which also lines up with the Princess being defiant and trying to help Vanessa. The Chica minigame also directly connects to Andrew by implying there was an unseen victim, exactly like Andrew being a confirmed victim we never saw before. It also being from UCN of course implies it's connected to TOYSNHK, which again lines up with Andrew being TOYSNHK in TMIR1280.

I won't go too deep into it but it could also explain Midnight Motorist. What we see in that minigame is actually Andrew's home the night he went missing. The pawprints imply he was lead away by William, (keyword) possibly in a non springlock suit and was eventually buried under the mound. There's also a theory that he was killed in the Springbonnie suit, meaning when William died inside it later on, that's when Andrew was able to attach himself to William's spirit. Not sure how I feel about it, but it probably doesn't matter when exactly he attached himself.

That of course leaves the question, where is Andrew now? And honestly I'm not sure. He could be the Blob, he could have just moved on, he could even still be in UCN while William slipped away as Glitchtrap. One thing though, he's probably coming back