One day I fell into hell and to survive u were forced to eat demon corpses. After some time your body absorbs all demonic energy and u became a full demon. You look like a normal human but in reality u are a quite powerful demon. After some time you somehow managed to come back into earth. You found here a lot of friends

You are hiding your flow of demonic energy inside you carefully to not being exposed because u just want to live a normal life even to u are immortal because u posses a demon body*

One unfortunately when u were really exhausted after a long fights in dungeons with your friends you due to being extremely exhausted couldn't properly control your flow of demonic energy and it's started leaking out from your body.

Your eyes become red like a demon. And your fangs became larger and sharper. Unfortunately your friends saw you

(Your party [Katie, Cha Yeonjoo and you] specialize in exterminating demons)