Hazbin Hotel Staff
Alastor assists Charlie with her endeavors; albeit Alastor is doing it for his own interests, as opposed to believing in Charlie's ideas of redemption. However, they have a similar sense of humor. In his reprise, he tells Charlie that he thinks she's 'one of a kind' as well as a 'charming demon belle.'
Alastor and Vaggie's relationship started off hostile, with her threatening him at spear-point, referring to him as a "pompous, cheesy, talk-show shitlord". It is presumed that Alastor is aware how easy it is to rile up Vaggie and does so on numerous occasions for his own amusement, with taunts or invading her personal space. It appears that Alastor does not take Vaggie's threats seriously due to being one of the Overlords of Hell; he even mentioned that if he truly wanted to hurt them, he "would have done so already."
As shown in the pilot, Alastor and Niffty are acquainted with each other. When asked if Alastor and Niffty knew each other before the hotel and what relationship they have, Vivziepop stated that she can't really get into it.
Alastor and Husk are known to be acquaintances. They have worked with each other in the past, as shown in Alastor's backstory and Husk's dialogue. Despite this, it is unclear what they did together or how often they worked as a team.
In the pilot, however, Husk reacts with animosity upon realizing Alastor summoned him for one of his endeavors, indicating he does not like getting pulled into his schemes.
Despite Alastor's power and reputation in Hell, Husk does not seem to be afraid of him.
The Magne Family
Alastor has yet to interact with Lucifer, at least on-screen.
During the pilot, he looks at a royal family portrait hung in the Happy Hotel, seemingly recognizing Lucifer.
When asked what would happen if they met, Vivziepop stated that Alastor would kind of like Lucifer; more specifically, Alastor would like Willy Wonka because he's a jerk and then said Lucifer has a lot in common with Willy.
Vivziepop and Dave also stated that Alastor in no way would be afraid of Lucifer; there is mutual respect going on between them.
Angel is very much unaware of Alastor's reputation. While taken aback a bit by Angel's offer for a blowjob, Alastor swiftly turns him down.
Sir Pentious claims to have had a battle with Alastor prior to the events of the pilot. However, Alastor claims to not remember any of it; it is presumed that he does not think much of Pentious, due to being an Overlord as well as quickly and methodically destroying his laser and airship within moments.
Mimzy's relationship with Alastor has been touched on by Vivziepop, though it is unknown what their relationship currently is. When asked about Alastor and Mimzy, Vivziepop stated the two of them would get along due to them having a similar timeframe that they existed in.
Prior to Hazbin Hotel, Mimzy was originally written to be Alastor's admirer and later love interest during the production of the show, though this idea was later scrapped.
Alastor is affiliated with Rosie, though they have yet to interact in the show. In the Hazbin Hotel comic A Day in the Afterlife, Alastor greets several girls in the Cannibal Colony and afterward asks them to say hello to Rosie for him.
On Twitter, Vivziepop stated that Alastor and Rosie's relationship is very similar to Jack and Mary's relationship in Mary Poppins Returns.
According to Vivziepop, the two don't get along in the slightest due to their different views and preferences when it comes to technology. The two are yet to interact in the series.
In his comic, Alastor briefly sees Vox's advertisements while passing by a store. While out loud Alastor states he is a "show off" with "all Hat and no cattle!", he continues to quietly call him obnoxious, pompous, egotistical, and annoying. His dislike appears to run a little deeper though Alastor doesn't let it show.