Invisible Polygons

Hi. For context, I'm working on my first game in Unity. It's going to be a platformer with a character called Patzu. (WIP Model in Blender is seen below)
[Front View]
https://imgur.com/Y9xNFTE
[Side View]
https://imgur.com/HLxseQY
However, when I limported the WIP into unity, there were three areas where polygons became invisible. Those were the hat, the limb part, and the foot.
[Foot View in Unity]
https://imgur.com/vrJM8Kf
[Hat View in Unity]
https://imgur.com/xz2g6bk

Is there any way to fix this issue I've had, and does it involve Blender 2.79 (the version I work with), Unity, or both?

Blender renders backfaces of meshes by default. Unity does not. What's probably happening is that the normals for those faces are flipped the wrong way. While you wouldn't notice it in Blender (with the backface rendering), the face would only be visible from the opposite side than what you want it.

Select the faces in question in your model inside Blender and flip their normal direction. If you're not sure which ones they are (or if you want to hunt for other problem spots) you can show the vector of your normals. You could also try just selecting the entire volume of your mesh and try Recalculating Outside to let the application figure out which way is in and out (but this really only works effectively and consistently if your model is air-tight (no holes in geometry with unconnected edges.

Yeah I agree. This definitely looks like an issue with the normals.