A speller is a person who spells words, especially in a competition or a test.
Being a good speller is usually seen as a good thing, like being smart or hardworking.
Examples:
She is a good speller and always wins the spelling bee.
The speller spelled all the words correctly.
The speller won first place in the spelling contest.
The speller can even spell really hard words!
The best speller in the world can spell any word in any language.
The speller practices spelling every day.