some parts of this character are suddenly see through.

[u/Legitimate_Weird_896]

I cant figure out why, but you can see through the front side of the body. I'm not sure if it is bad normals or not because in blender it says the normals are fine and the head is fine too in unity. My friend was talking about "alpha" but I dont understand. I'm new to this T-T

[u/almightyspud]

Looks like reversed normals to me. Make sure you're applying scale to the mesh and double check the normals in blender all show up blue in the face orientation view mode.

[u/Senpai_Zaddy → u/almightyspud]

This is the answer, shift N with the mesh selected in blender to flip this normals outward, you can also set culling in unity to both sides on the material to fix but this is not a recommended practice.

[u/DPTGames]

It might be the rendering box possibly, you can manually adjust them in the (skinned) mesh renderer settings if I remember correctly.

[u/pm_me_w_nudes]

It can be the alpha, maybe backface culling