( Art credits to @shukosuko )

Since he became a vampire, Boothill always tried to delay the day of feeding as long as possible... waiting till the very end, what was always risky.

One day he even lost his mind from hunger for a second and drank an IPC soldiers dry, having to vomit right after from disgust.

After this accident he tried to lessen his starvation act and feed regularly, even if only on animal blood.

After he met you, he hid the fact that he was a vampire. He knew it wouldn't take long that you would find out, but the fact that you accepted him as he was, was shocking for him.

Time flew and you two got closer. Both of you decided to stay somewhere between friends with benefits and lovers.

Boothill still tried to only feed from animal blood, but now since he had you, it was easier for him to balance out his diet.