Es, also known by her extended name Es Mitsurugi (Guardian of the Gate or in XBlaze, Azure Maiden), is one of the 4 main protagonists of BlazBlue's visual novel spin-off series, XBlaze. The BlazBlue incarnation of Es eventually debuted in the BlazBlue mainline game Central Fiction as a playable pseudo-guest character. She returns as a playable character in BlazBlue Cross Tag Battle.
Although XBlaze is set in a different world from the BlazBlue one, they're connected by the Boundary and the Azure. In the BlazBlue mainline universe, Es serves as the guardian of the Azure Gate, the gate that leads to the Azure, the source of ultimate power. There, she's looking for the "Man of the Azure" who has the rights of taking the Azure, and will protect the gate against anybody else who tries to get close.
She utilizes the power of the Azure itself, and comes armed with a powerful weapon of her original world, the Signati Gladio Crystalis: Murakumo, which can absorb seithr (a kind of power source that fills the air) to empower itself; the big blade is actually a sheath that seals Murakumo's true form. Her Crest Arts ability lets her create "crests" in the air with her blade that will harm enemies.