I'm new to Unity.
I started modeling a few things in blender to import them in Unity and see how it works, but they're always missing some faces and looking weird.
Anyone knows how I can fix this?
First guess would be that some surface normals are not pointing towards the correct direction. When you export geometry it usually exports one-sided geometry which means that you have to be careful which way your normals are facing (it only renders one side of a face; when you look at the face from the opposite side, it doesn't render). This could explain your problem. Check blender documentation for normals/surface normals.
It seems that the normals are flipped. It is a 3d modeling problem where your polygons are facing the inside of your object instead of outside. That must be solved in your 3d modeling application (3ds max, maya, etc) and the model must be reexported.
if a poly is not a quad then you can get problems.
If it's not an issue with normals, another thing that can cause this is if you are doing bones and animations in blender and somehow miss assigning bone weights to one or more vertices.
You can easy check if a normal is facing the wrong direction. In Blender select your Mesh >> Go in Editmode >> Search in Panels for Tab "Mesh Tools More" >> Here you hit "Draw Normals" button. Now you can see a turquoise line in the middel of every face.
I found a solution in Blender. in “Edit mode”, select the whole model, click Shading / UVs tab, and click “Recalculate”. This will flip the needed models to the correct direction.