Sisir

Sisir is the titular antagonist of every FNASH game to date, except for Garrett And The Meme Machine. He is the creator of almost every monster seen in the games.

Before FNASH 1
Before FNASH 1, Sisir was the creator of the Karma Discord server, which all of his friends (and therefore almost all of the Monsters) were in.

In FNASH 1
In FNASH 1, Sisir takes on the appearance of Monika from the Visual Novel Doki Doki Literature Club, except her face is repleaced with Sisir's. He acts the same as the other normal monsters.

In FNASH 2
In FNASH 2, Sisir's appearance is changed. His body is now a Xenomorph, but he has the same face. He acts the same as he did in FNASH 2, but in the Night 5, instead of Computer Guy, he talks to you.

In FNASH 3
In FNASH 3, Sisir appears as a Ruined Monster. He has extra arms, and is fragmented like the other Ruined. Instead of acting like the other Monsters, he climbs through the Air Vents, and can only be stopped by turning on the Fan of the Vent area he's currently inside.

In Garrett And The Meme Machine
In GATMM, Sisir has a new look. He is no longer a Xenomorph, and his body has been replaced with Springtrap from Five Night's At Freddy's 3. He sends Garrett to the 7th Circle to find gears for his titular Meme Machine. He speaks to Garrett via the Walke Talkies.