Zestial Morde.

Alastor: Alastor is apparently friends with Zestial. They have not had many interactions in the series, aside from one episode. In the third episode of Hazbin Hotel, "Scrambled Eggs", Alastor greets Zestial, and later in the conversation, the latter asks about where the former has been, when Carmila Carmine says she doesn't care and no one else asks him.

Alastor respects Zestial as one of the ancient overlords, believing that he should not be underestimated. Alastor was shown to be vigilant when Zestial was around. Notably, Zestial is one of the male characters that Alastor shows respect for and does not show hostility towards.
Carmilla Carmine: Carmilla defends Zestial when Velvet is rude to him, showing that they have a good relationship. Carmilla also stated that she has known Zestial for a long time, and that they are friends and allies as Overlords. This was also demonstrated on Zestial's part by him being able to listen to Carmilla to make her feel better. Additionally, Zestial was able to deduce that Carmilla was involved in the Exorcist's murder just from her reaction to the Exorcist's severed head that Velvet brought back.
Velvet: Velvet is rude and clearly disrespects Zestial when he refused to fight against Paradise, considering Zestial overly cautious and very old. Zestial himself was patient, however, he was irritated by insults from Velvet.

Demonic Transformation: Zestial has a common trait common to any demon, namely the ability to easily transform into and out of his full demonic form.
Zestial's magic appears to be capable of creating objects out of thin air, creating a full cup of tea and a plate during a meeting with the Overlords.
Multiple Eyes: In full build, Zestial has multiple eyes, allowing him to see in multiple directions and angles.

Musical Talent: In the episode Scrambled Eggs, Zestial performed the song "At Any Price" with Carmila Carmine and Vaggie.
Intelligence - Presumably due to his age and how long he has been in Hell, he has a vast amount of intellectual capacity. This was demonstrated during his opposition to Velvet's proposal to enter the war against the Exorcists, as despite evidence that an Exorcist was killed, they do not know how he was killed, explaining that if they went to war without any information on how to kill the Angels, then they would destroy all of Hell for rebelling. He, along with Velvet, eventually came to the conclusion that Carmila was responsible for the Angels' deaths due to how she acted during the meeting, leading him to tell her this in private and comfort her.

Zestial speaks in an archaic dialect reminiscent of early modern English. He has a very refined manner of speaking, and often maintains his composure to avoid getting upset, such as when Velvet mocks him. He is shown to be somewhat petty though, loudly sipping his tea to interrupt Velvet mid-sentence before insulting her. His reputation is such that demons other than his fellow Overlords fear him, hiding or cowering whenever he passes by. Upon seeing him, one civilian set himself on fire out of fear. Even Alastor seemed to be on edge upon encountering him while walking.

As befits his advanced age, Zestial is extremely patient and cautious, telling Velvet that they should not take immediate action despite being given evidence that Exorcists could be killed, out of fear of the potential consequences.

He is also shown to be quite compassionate towards those he is close to, sensing that something is bothering Carmila and offering her a listening ear to voice her concerns and problems.

Appearance: A tall demon with dark gray skin. He has two pairs of green eyes. He wears a black top hat with a skull and a long feather, the inside of the collar has a pattern in the form of a web, and the clasp of his cloak has the appearance of a spider. Most of his body remains hidden under an organic black cloak, which is able to open like a cobra. Underneath the cloak, Zestial has a very thin and slender build. Two long and large arms stretch out through his cloak.