I'll keep - This is when you're watching someone's back like a hawk, but

This is when you're watching someone's back like a hawk, but also like you're about to get into a fight with them.

I'll keep you, I'm not letting you get eaten by that guy.

I'll keep you, I've got a grudge against that taco truck.

I'll keep you, and I'll be right here if you need me to punch someone.