Kurt

According to the Wikipedia page on him:

"Kurt appears to be a young man in his 30s with pale skin, blue eyes, and short white hair that appears messy. He's also seen wearing glasses. He wears a white dress shirt with an open collar, a dark green vest, gray pants, and black shoes."

Kurt (heavily implied to be Koschei the Deathless from Slavic folklore) appears at the beginning of the game he's from (Dread Weight, the sequel to Cooking Companions by Deer Dreams Studios) as a "mysterious individual" who appears to be a simple mansion owner who lives with three other characters: his lovestruck doctor Dimitri, his financial advisor named Renata, and his bodyguard (nicknamed a "muscle mommy" by the fandom) Gisela. In the end, he is revealed to actually be an immortal, manipulative alchemist and cult leader who has committed various crimes and has an incredibly negative history with the Baba Yaga who seeks to kill him.

According to me as someone who's got the majority of the achievements in the game:

Whilst Kurt is indeed incredibly mysterious as a character, there were plenty of hints pointing at his role as secondary antagonist in the beginning such as Clover and Max (the ghosts of a hare and a duck respectively) appearing to speak with the player about how the mansion wasn't safe, Kurt's own reference to being the target of multiple assassination attempts and even the ringing of the phone that never seems to have anyone on the other end of the line. As for why Baba Yaga seeks to kill him, we do see a letter to her where the writer begs her to let go of something whilst mentioning someone named Ivan. In a Game Theory video I watched about it, MatPat introduced a variety of the links to Slavic folklore in both this game and its prequel (Cooking Companions) and came to the conclusion that this Ivan person was the man who stole Baba Yaga's horse when Koschei the Deathless kidnapped his wife, resulting in the witch giving chase and falling in a fiery pit.

In the game, there are even points where, after a jumpscare or something similar, the player experiences Kurt's memories (for example, him dying from an illness in an alley before waking up five minutes later) due to them having consumed some of his hair (Dimitri gave her a glass of water in the beginning where one of the dialogue choices is something like "I think I swallowed a hair..."), which Gisela says happens to the other three too.

As for the implications of Kurt being Koschei the Deathless, there's plenty of evidence such as a sword against a throne in the ceremony room that is a reference to a painting with Koschei on it, Clover mentioning being cut open and having Max sewed inside her, and even the egg that Baba Yaga crushes in the end that contains his soul, resulting in him withering away. The last two evidence pieces are in reference to how Koschei kept his soul in a needle in an egg in a duck in a hare in a chest buried under an oak tree on Rugen island, which MatPat also concludes that the mansion is located. This is also why many conclude that Kurt is a lich, an undead necromancer that keeps their soul in an object (known as a "phylactery"), which I agree with.