Yours, Etc.: Origins and Uses of 8 Sign-Offs
"visitor," mid-14c., agent noun from come. Meaning "one showing promise" is attested from 1879. Phrase all comers "everyone who chooses to come" is recorded from 1560s.
A person doing very well and promising to do better in a certain field: She's a comer, a potential champ (1880s+)