IMHO I think that's her hair, look at the artwork x:
And knight makes more sense for power imo. Everyone knows charms can transform into beasts and it makes more sense that the beast guardian be for charms. It seems really odd still imo and I hope they change it personally :/
And wasn't there an image earlier where a champion had the knight guardian standing over it?
Diablo III North America Glacea#1307
League of Legends Queen Nimera, Level 20
New Leaf/Pokemon XY 4511 - 0825 - 1589