supporter; cheerer; rooter
A person who supports or cheers for someone or a team, often at sporting events or competitions.
He is an enthusiastic supporter.
Many cheerers shouted encouragement at the match.
Fan; implies a more personal or emotional attachment to a team or celebrity, while 応援者 focuses on the act of cheering.
Compound of 応援 (ouen, 'support, cheering') + 者 (sha, 'person').