The season 3 premiere of What We Do In The Shadows hinted at the real reason why Guillermo chooses to remain as Nandor's familiar — despite it being unlikely he'll ever be turned into a vampire. The relationship between the two is that of a long-suffering servant and a naïve master; however, after over a decade of exemplary service with no apparent reward, there must be a reason why Guillermo sticks around.
Guillermo first became Nandor's familiar in order to fulfill his lifelong dream of becoming a vampire.
Despite his dedication over the years, whenever the subject is brought up, Nandor shrugs it off. The idea is a running gag with What We Do In The Shadows' vampire characters, who so rarely fulfill that often-made promise to their familiars — instead, typically exploiting weak-willed humans indefinitely with insincere promises. But Guillermo is smart enough to have recognized that, meaning there must be some deeper reason why he chooses to remain at Nandor’s side.
It's become clear that the reason Guillermo sticks around isn't that he's gullible or thinks he'll become a vampire — it's because he is in love with Nandor. The idea has been touched on before in the show, but What We Do In The Shadows season 3, episode 2 specifically positioned his feelings for the vampire as romantic. Guillermo talks to Meg about his relationship with Nandor, and because he's disguised as the vampire, frames the problem from Nandor's perspective; however, Guillermo's genuine feelings color how he talks about his master, leading Meg to correctly guess that he (Guillermo) is in love.
Guillermo puts up with mistreatment, and won't permanently leave no matter how bad his experiences at the vampire house are, because of his romantic feelings for Nandor. What We Do In The Shadows season 3 confirmed this through the conversation with Meg; however, this revelation was set up by previous events. When Guillermo temporarily left his vampiric master in season 2, the scene read exactly like a breakup. Later, Guillermo allowed himself to be wooed back by Nandor with the promise of considering turning him into a vampire — but this is only an old habit now or maybe even an excuse. The real reason Guillermo was happy to come back was because Nandor demonstrated a desire for having him back. At the end of the day, Guillermo just can't stay away from Nandor for long.
Nandor and Guillermo's power dynamic and relationship have changed over time. While Guillermo used to be subservient, he now freely talks back to Nandor, who has shown a gentle side to (and possible affection for) his familiar. Something that has recently helped the two be on equal footing, allowing romantic feelings to surface, is they are now equally dangerous to each other: Guillermo is growing into a talented vampire slayer, thanks in part to his ancestry (and foreshadowed by his last name, De La Cruz). It will likely take Nandor a long time to realize or even reciprocate these feelings; still, it will be interesting to see if Guillermo and Nandor do develop a romantic relationship in What We Do In The Shadows.
What We Do In The Shadows releases new episodes Thursdays on FX and Next Day FX on Hulu.