Masakado is a powerful Persona of the Tower Arcana. Unlike most Personas, you'll need a special Key Item in order to unlock its Fusion. If you've been consistent with the Requests from Elizabeth, then you should have no trouble unlocking Masakado. If you're going for 100% Compendium completion and intend on turning in every Request in Persona 3 Reload, then you'll need to get Masakado during your playthrough.