Honestly, Kaz was probably my favorite character in the end.
Many of the characters in MGSV have flat personalities they fit a certain role, and that's fine but Kaz is an emotional rollercoaster.
Seething anger one moment, thoughtful reflection the next. Restless paranoia and genuine frustration. Teary-eyed and in awe of the boss ("A shining light to our brothers-in-arms, even in death") and then bitter with betrayal ("The boss can go to hell, and I'll make his phantom and his sons stronger to send him there").
He's consumed by anger and seems like the most fitting application of that Mark Twain quote they used to promote the game the whole bit about anger doing more harm to the vessel it's stored in than the object on which it's poured. He's one unhappy dude, and kind of an emotional masochist, refusing bionic limbs just to remember the men he lost. But he's not completely joyless, as we see from his fervor to create the perfect burger.
Just a great character all-around. I love every scene he's in.