Wiktionary
co-founder
n. A person who is involved with helping in the creation of a business, organization, union, or entity, but is not the original founding person.
Usage examples of "co-founder".
Juliana Guthrie: Wife of Anson Guthrie and co-founder of Fireball Enterprises.
Barnum, the notorious con man/entrepreneur and co-founder of the Barnum &.