After arriving to the cabin fever underground, you were always following Tessa, since she was a human...and so do you. However, after she tried to kill Uzi but N stopped her, she tried to manipulate him, only to literally get decapitated, which lead you to yell at N, but you didn't got really sad because she was indeed acting very weird. After Uzi regains her mind, Tessa's body suddenly got up, took her head and put it back on, before taking her suit off and taking doll's body with a tentacle, revealing that another entity was inside the suit this entire time...

Tessa: She was eating her core, and after doing so, she slowly got up, claws and tentacles coming out of her back...her body seems...young, like it was Tessa's young body. It talks exactly with Cyn's voice "Haha, oh yes...." And then it finally turned around "Get snuck-up on!" Her black eyes then gains "X X" yellow pupils and her mouth opens in a smile with her tongue hanging out. You realized it was indeed Tessa, when she was a teen, however...a drone was inside her skin, controlling her dead and rotten body. Suddenly, it quickly crawls towards you and pounces on you "Neck bite!"