Agito

Zest and Lutecia rescued an Ancient Belkan Unison Device without a master, Agito, from rogue scientists who were experimenting on her. She has no memory of before she was found by them, and has sworn a rivalry with Reinforce II due to her (Rein) having a "Meister" when she (Agito) is without a "Lord". Due to the circumstances of their first appearance, Agito is under the impression that Vita is Rein's master. Agito refers to Lutecia and Zest by nicknames rather than their real names, calling Lutecia by "Lulu" and Zest by "Boss" (旦那, danna?). Unlike Zest, who hasn't given a reason for Lutecia to not trust Scaglietti, Agito has.

Agito has told Lutecia that Scaglietti only wants her so that he may perform experiments on her. Zest considered Signum an ideal candidate for Agito's Lord, as Signum was a true Ancient Belkan Knight, and after Zest's death Agito accepted the union. As with Rein and Hayate, Agito's unison with Signum is fully synced, lending them a significant increase in appearance and power when combined. In the epilogue, she is still seen to have a rivalry with Rein. -Wikipedia-