challenger
A person who challenges someone or something, often in sports, competitions, or debates.
彼はチャンピオンに挑戦者として立ち向かった。
He faced the champion as a challenger.
次の挑戦者は誰ですか？
Who is the next challenger?
挑戦者 is the challenger trying to take the title, the natural opposite role to 王者, the reigning champion.
Compound of 挑戦 (challenge) + 者 (person).