This game object has a mouth where I would be able to see the insides of it through it, but in this case for some reason I can see behind him (floor, walls, and whatever object it’s behind him)
This is what I should be able to see:
Tris in game engines are one sided, while some 3D software treat them as double sided by default.
The best solution is to model the interior as well. Other than that look into how to turn culling to off in the shader or turn on double sided.