Emerald City Comicon is in one week. Cosplay panic aside, I have a spandex issue that I need help with. I need to keep the collar from rolling back. As you can see in the images below, Korra’s collar fits snuggly around her neck. Because of the way the collar splits in a “V” at the center front, the corners of the collar are prone to curl back.
I know that interfacing exists for knits and spandex. Is that the best route? Should I stabilize the seam lines and the white trim? Or adhere a section of interfacing to the full front collar section?
Thank you for your advice!
The Fabric Alchemist 
I don’t really know anything about stabilizing knits, but I bet you could rig something up using clear elastic and snaps or hooks, so that it could pull relatively snug against your neck without hindering your ability to get in and out of the costume.
I wonder if some sort of light/thin/flexible boning in the binding could work? Or perhaps the binding could be lightly-stretched FOE?
FOE?
Ha, sorry! Fold-over elastic. You see it a lot on knit camisoles and undies.
I haven’t sewn with knits before, but boning sounds like it might be a good idea. If you have an old mens shirt that you can cut up, they often have thin plastic boning in the collar. I’m thinking these would be just right for this project.
Otherwise, would it be possible to use a stiff non-knit for the white stripes? Or would that look weird?
Yep, as others suggested, plastic boning (steam iron it flat in its casing!) or collar stays like in the fancier men’s dress shirts (you can buy packs of replacement collar stays). Interfacing might help slightly, but you need more structure than that.
Adding the white trim helped stabilize a bit. I ran out of time and didn’t try adding plastic boning, but I bought some!