Xerxsephnia von Blitzengaard is a sister of Martellus von Blitzengaard (AKA Tweedle). She goes by the nickname Seffie. She has been at least an acquaintance of Gilgamesh Wulfenbach since they were children.
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.
Despite these traits, she shows Martellus enthusiastic affection, hugging him and announcing that he is her favorite brother and she's glad he's alive. 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.) So far, unsurprisingly, Gil has resisted this proposal, but he has admitted, to himself at least, it is a good idea.
As noted above, she makes herin Paris, greeting Agatha as the latter woman (and her unwelcome tag-along Tweedle) arrive in the city, inviting her to a family party, and almost physically dragging Tweedle away to give Agatha some space, to get him a change of clothes and to take him to meet Grandma.
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.
Apart from her rather.. intense.. attitude towards Gil, her ultimate goals and true loyalties remain open to question; during the chaos that erupts during the aforementioned ball, she doesthat keeping Tweedle alive, at least for now, is part of her "new master plan."
- ↑ "..she is your friend - as much as anyone from a rival house can be.."