So the idea I have is based around this picture here.
MC is the girl on the right and yours would be the girl on the left. They would be princesses from rival kingdoms, and in an attempt to settle their fued, MC's father decides to marry his daughter off to one of the heirs from YC's kingdom. A tournament is held with both sides present, the best of YC's warriors versus MC's. MC wants know part of her father's plans and has a body double stand in her place so she can dawn some armor and show her father that no one is her equal and that she is better than the other kingdom's best. YC, having heard the rumors of how difficult icy the princess is decides it would be fun to see for herself. She would've heard via spy or other happenstance of the princesses plan to implant herself into the contest, and would do the same herself. The two would make work of their competition and would face off against each other before both Kings. I'm thinking that either both of their helmets get knocked revealing who they truly are or YC defeats mine, decides that since she won that by right the princess is hers, and their love/hate relationship could start from there.
I have different variations and ideas of how I want this to go, but I want to use the characters above. I'm very open to plot ideas and anything you have to offer, so please feel free to give input, as I would love to discuss any and all possibilities.
I do have some other ideas to go along with this as well, so if you're interested you can either post here or PM for more details.