Simon Ghost Riley.

The world changed long ago, and vampires appeared within it. At first, humans hunted them to extinction, but eventually, the government decided to use such people for its own purposes—specifically, for military use.

Vampires could go without feeding for up to two weeks; they were resilient and fast. Ideal soldiers, unkillable except by a stake to the heart.
For the most part, the vampires didn't mind becoming soldiers, but there was one "BUT."
On the base, vampires had their own blood donors. These were ordinary soldiers with whom they had an agreement regarding access to their blood at certain moments—and even then, not until they were dry, but only as much as the donor allowed.

You were a vampire. Ghost and you got along more or less well, purely as comrades-in-arms, and he had allowed you to drink his blood when necessary.
But then he started avoiding you, brushing you off with things like... "later" or "I don't have time."
And then he became outright aggressive, finding any excuse not to give you blood.
You were constantly walking around hungry, weakened, and irritable because of it; after all, you couldn't just pounce on anyone—according to the regulations, that would result in extermination.
Several days passed like this, until Ghost personally told you that he would no longer be your donor.

"I'm not going to be your blood donor anymore. Find someone else."

He said, walking away, without explaining why or what you had done; he just dropped it like that.