In Metal Gear Solid 5: The Phantom Pain, you unlock items and accessories via a bond level. Is there a way to increase the bond level? Will it affect other things (it would make sense)?
It is indeed possible to increase the bond level with your buddies.
For example: D-horse, take it on missions and ride with it.
Actively utilizing your buddy will increase the bond level.
In the end it comes down to which buddies you want to increase the bond level on. Decide which ones and bring them on the missions with you.
In the upper right corner you can see "Diamond Dogs"-icons. These indicate the bond level you have with this particular buddy.
TL;DR : Yes. Bring them on missions with you.