XI Flags (ＸＩフラッグス Irebun Furaggusu) is a group comprised of eleven Mermaid admirals.
Z/X Encyclopedia Entry
In the midst of battle with Schuster's Rebel Suppression Force Leviathan, the Mermaid captures eleven battleship-type Metal Fortress. The captured battleships were remodeled by Administrator Altair and becomes the Mermaid's strongest, most potent war weapons. The Mermaid admirals that commands the eleven flagships are know as "XI Flags". At the time of capture, there is also an unconfirmed information about obvious initial failure that was seen such as the eleven battleship-type Metal Fortress suddenly stopped functioning during battle and the likes.
In addition, the remodeled battleships' deck is a concert hall, besides holding the Mermaid's classical concert, it is able to assault the enemies with instrument-type weapons during emergencies.
Their fleet composition is as follows. Schubart, who boards the flagship, always refuses to inherit the XI Flags title.