Zantabraxus is the current Warrior Queen of the hidden city of Skifander, along with being the of Zeetha and (probably) the wife of Chump. The comic proper hasn't actually said much about her or how absolute a ruler she is; pretty much everything we know has been passed along via scattered comments from Zeetha,
Considering her appellation and the emphasis Zeetha has placed on battle training, Zantabraxus may be the best fighter of her people. Upon securing the throne, she chose Ashtara as her patron goddess. At one point she a serious internal threat to her reign and had to go into temporary exile. She used to on personally slaying the local Fafflenarg Beasts instead of doing the whole "delegating of responsibility" thing. Unlike her distant predecessor to the throne, Luheia, she is not unaging and immortal.
Following the surprising arrival in her skies of the ill-fated Europan airship expedition led by Professor Consolmagno, Zantabraxus had her daughter home with the visitors "to see what the rest of the world was getting up to." We've seen two versions of how her meeting with the good professor went. This is probably down to artistic shifts on the Foglios' part over the years rather than being plot points.
Unique to the Skifandrian women is the color of their hair, which being green may have saved expedition guest Zeetha from being murdered out of hand by the sky pirates who captured her while she was sickIn her only full-color portrait to date, (which is not in the comic proper, see below) Zantabraxus is shown with dark green hair, which may be artistic convention or perhaps a regal trait.
Zeetha has told Axel Higgs that as her chosen beloved, he has to fight Zantabraus.
When Klaus first Zeetha and has an animated exchange with her in the Skiff tongue, his last word is "Zanta--!" just before he is cut off by the Spark-infecting slaver wasp, leading attentive fans to the (very reasonable) conclusion that he was about to invoke the name of the Skifander Queen. It has been essentially confirmed by the Foglios (though not in-strip) that "Chump" is a/the name used by Klaus while he was in Skifander. This makes it almost certain that he and Zantabraxus are husband and wife. At the very least, the two of them had adventures together, and he helped her reclaim her throne following the aforementioned exile.
Zantabraxis in the novelizations
The print novel Agatha H. and the Voice of the Castle revealed/confirmed Her Majesty's name and specific relationship to Zeetha long before the comic did, albeit with the spelling Zantabraxis.
At one point in Agatha H. and the Clockwork Princess, Klaus mentally notes that Bangladesh DuPree's preferred interior-decoration scheme, namely bladed weapons, emerald paint, gold trim and lots of animal bones, brings back fond memories of his wife.
Zantabraxus in The Works
One card in The Works card game is identical (or very nearly so) to the Queen pictured in the first description of Zeetha's adventure: Zantabraxus, who has the labels Queen and Spark.