What is a word that describes a person who takes up a challenge

The same thing as the title. The situation is a person whose intent is against the protagonist's and is taking up a king's challenge.

I'm rewriting a fairytale from a different perspective. The fairytale being The 12 dancing princesses.

This has been answered. Thanks for your help!

Solution 1:

I would say a champion. A champion loves being challenged because that's the way he gets to prove he is still a champion.

Also try adventurous, bold or daredevil.

Solution 2:

I would say "a contender" - a person or group competing with others to achieve something.

Contender — M-W

a person who tries to win something in a contest; especially : a person who has a good chance of winning

"There are several contestants, but only two real contenders."
"This latest defeat means that she's no longer a contender for the world title."