Rhombulus has a very short temper and will quickly engage those he thinks have insulted him. He is also somewhat immature, typically being disciplined by being put in a time-out corner and threatening to crystallize himself because he thinks no one would miss him. As demonstrated in "Crystal Clear", Rhombulus is also very impulsive and has a tendency of acting without thinking things through, which has resulted in him suspecting many creatures of being evil and imprisoning them in crystal, some of whom may be innocent.
Rhombulus' snake hands have minds of their own, and they sometimes act as the voice of reason for Rhombulus' reckless ideas. Unfortunately, Rhombulus seldom listens to them.
Rhombulus first appears in "Page Turner" as a member of the Magic High Commission with Hekapoo, Chancellor Lekmet, and Omnitraxus Prime. He and the other council members call Glossaryck into their office at the Bureaucracy of Magic, and he almost immediately gets into a conflict with Glossaryck when Glossaryck calls him a child. For his immaturity, Hekapoo puts him in the time-out corner.
In "Crystal Clear", Rhombulus captures Star Butterfly under suspicion of sapping magical energy from the universe and brings her before Lekmet. However, Rhombulus has no hard proof of Star's guilt. Before Lekmet can inform the other members of the Magic High Commission, Rhombulus freezes him in crystal. With a situation spiraling out of his control, Rhombulus makes an unlikely friend in Star Butterfly, who encourages him to think things through more without relying on his gut intuition. She also reveals to him that she lost Glossaryck and her book of spells to Ludo, and he promises to tell no-one about it.
In "Face the Music", he and the rest of the Commission attend Star's Song Day ceremony. He cheers for Star during the song, and later becomes flustered when the rest of the Commision admonishes Queen Butterfly for keeping them in the dark regarding the loss of Glossaryck and the book of spells (which was revealed in the song's lyrics). In "Starcrushed", Rhombulus takes part in a mission to recover Glossaryck and the book of spells from Ludo, and he engages Toffee in battle, but Toffee absorbs his powers.
In "Return to Mewni", Rhombulus is stated to be in stasis, and Star and Moon bring his and the rest of the Commission's bodies to the Magic Sanctuary in an effort to restore them. In "Moon the Undaunted", he appears in flashback as a member of Queen Moon's royal court. In "Rest in Pudding", he attends Lekmet's funeral, fondly reminscing on happy memories with him.
In "Stranger Danger", he and the rest of the Commission capture the freed Eclipsa, and examine Star to see if Star was affected by Eclipsa's "evil". In "Monster Bash", he leads a raid on the monster temple during Star's party. In "Total Eclipsa the Moon", Rhombulus and the rest of commission are seen rushing to the Bureaucracy of Magic's archive room after Moon accidentally triggers the alarm.
In "Butterfly Trap", Rhombulus takes part in Eclipsa's trial, where it is revealed he and the rest of the High Commission orchestrated a conspiracy to replace Eclipsa's daughter Meteora on the throne with the peasant girl Festivia.
In "Divide", Rhombulus and the Magic High Commission try to serve as advisors for acting queen Star, but Star immediately dismisses them for their deception toward the Butterfly family. As they depart, Rhombulus petulantly states that Star will need them.