We know, in chapter 3, that darkners are objects. Ralsei himself claims to be a darkner, so if we choose to believe him, he has to be an object in the light world. Now, theres a big mystery on what the fuck is ralsei, with people theorizing on him being kris' toy horns, crayons and all sorta stuff. however...

In chapter 4, we see a sprite of ralsei with his face dirty with blood, after susie injuries herself and fucking wipes it off on him. Because of this, we now know that ralsei's fur can absorb blood to some extent.

Heres the deal: Ralsei is frequently described as being fluffy, and even in death he turns into a pile of floof. One of the fluffiest things aroud is cotton. And cotton is commonly used to clean absorb liquid, including blood. Because of this, tampons are frequently made of cotton, due to it's blood absorbing properties.

If we consider that whatever object ralsei is is made of cotton, and that he is aways present in the dark worlds that kris is in, we can assume that somehow kris is carrying ralsei around with them. And one of the only reasons one would carry around cotton everywhere, is in the form of tampons.

This theory confirms that kris is afab, potentually on their period during the events of the game, ralsei is a tampon, and is being carried around by kris.

Wether or not you think kris has ralsei in their pocket or is actively using ralsei during the game is up to you.