Xerxsephnia von Blitzengaard is a sister of Martellus von Blitzengaard (AKA Tweedle). She generally goes by the nickname Seffie. She has been at least an acquaintance of Gilgamesh Wulfenbach since she "was a child."
In terms of personality and tactics, she is more like her cousin Tarvek than her direct and brutal brother, being astute in political machinations and generally sneakiness. She likes pretending (or "pretending"?) to be evil or maniacal; her introduction page has her walking in ominously, exclaiming "the fools!" and mentioning she practices her creepy laugh but the harmonics aren't there yet.
She nevertheless shows Martellus enthusiastic affection, hugging him and announcing that he is her favorite brother and she's glad he's alive. (Later actions show that she's evidently as sincere about this as a member of her family gets.) Tweedle appears to genuinely like her in return and has openly praised her intellect, making her quite possibly unique among his family members. Further interactions between them show that she also has no hesitation about telling Martellus whenever she thinks his ideas are stupid and offering alternatives, such as when sheand lets Martellus chase Agatha alone so she won't be closely associated with him and can come across as more likeable to Agatha.
In the two and a half years that Tweedle has been missing, she and the still fairly mysterious Grandma have been engaging in political negotiations with Gil, and (deservedly or not) Seffie has earned Gil's trust to the point that he is shown allowing her to to several unknown but evidently important personages. She has also repeatedly promoted the idea of the two of them marrying, portraying it as a cold-blooded way to enhance their mutual diplomatic standing. (Her on the subject are revealed when she is later shown intercepting Gil's mail in Paris, something she has been .) So far, unsurprisingly, Gil has resisted this proposal, but he has admitted, to himself at least, it is a good idea.
Despite all this, there has been no indication so far that she is a Spark.
She has some sort of  with Colette Voltaire, and thanks to the latter woman's "suggestion", Seffie finds herself having to frantically organize an elaborate masked ball to mark Tweedle and Agatha's arrival in the city. During the chaos that erupts during said ball, she that keeping Tweedle alive, at least for now, is part of her "new master plan." Following the thwarting of The Other's immediate attempt to take over Paris, she is among those who are (politely but firmly) ordered to leave the city by its new Master Colette. In the process of this, she gets Mister Obsidian to Tarvek, and has the latter man the pirate-ish Jeanne Hawkins' airship The Mopey Tortoise and sent off to England after Agatha, hoping they will permanently pair up thus removing one major obstacle to her snagging Gil for herself. She states, evidently , that she is doing this in covert defiance of Grandmother's desire to keep Tarvek under her thumb.
She next Sparafucile, and one other unnamed individual from Captain Hawkins, who has returned to the Continent in triumph following the (eventually) successful completion of her previous run. They travel to Londinium, where Seffie urgently with her hireling Mr. Quinster to get in contact with Agatha. He is able to insert his employer's little band into the locked-down undersea dome of Queen's Society, whereupon Sparafucile kidnaps Agatha during an unguarded moment and brings her to Seffie, who is hovering near a large coffin-like bit of tech. When opened, it is revealed the inhabitant of this object is a naked Tweedle, unconscious and looking rather the worse for wear. Seffie slaps Agatha's hand on Tweedle's chest, which causes him to revive (somewhat); it turns out his tinkering with Agatha's body-chemistry affected him as well. A relieved Seffie immediately ; Sparafucile comments she has not slept in days. When in the chaotic course of events the dome is set to self-destruct, Seffie is loaded aboard the relevant escape vessel by Sparafucile, still sleeping.in Calais, a coastal port in France, purchasing emergency and highly covert passage for herself, her other Smoke Knight attendant
She eventually wakes up, and returns with Agatha and Co. to more civilized climes. In the process of this, an an ill-timed lurch new fashion sensation in English high-society. Along with everyone else, she attends a big shindig thrown by Queen Albia; it's a hit-or-miss affair for Seffie, while she ends up owing favors to both and , she does at least finally, after much effort and frustration, manage to a dance with Gil.which leads to her accidentally creating a
Tweedle then co-opts her into helping with his latest project, creating an Agatha-touch backup in the form of an uplifted cat. Before long, however, Seffie's nurse/etiquette teacher duties get interrupted by the fact that Aliba is shipping Gil back to The Continent to deal with a Polar Ice Lord invasion, with Seffie's unknowing rival Trelawney Thorpe in tow. Seffie of course joins them, under the essentially-legitimate cover-story of being able to help Gil politically.
Even after all this, apart from her rather.. intense.. attitude towards Gil and her affection for her brother, her ultimate goals and true loyalties remain open to question.