New Transfer Student; What is Guilmon Bread?

The Mane 7 get strange devices when a new teen shows up in Canterlot, who happens to have a similar item. However, strange creatures start appearing, causing destruction as they rampage. But the new kid shows up with a similar creature as well, fighting alongside them and swiping cards while shouting "Digi-Modify." Just who are they and how can they tell which Digimon are good or want to enslave all of humanity?

My Little Pony: Equestria Girls belongs to Hasbro
Digimon: Digital Monsters belongs to Bandai