Shio - She's a clerk that works in the Jack Frost themed Hee Ho Mart. Her appearance is a reference to Anna from the Fire Emblem series.
Marth- He's important aspect to the main storyline, although he didn't physically appear, but he helped out by reviving Itsuki, who was hit by Medeus's attack by placing his own Performa inside of him temporarily become one with him for final battle of Medeus.
