Cap'n Cuttlefish did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Cap'n Craig Cuttlefish is an elderly Inkling of the Splatoon series. Cuttlefish was a member of the Squidbeak Splatoon and fought in the Great Turf War between the Inklings and Octarians. After the Great War, Cap'n Cuttlefish moved to the outskirts of Octo Valley, keeping an eye on fishy behavior.
After the Great Zapfish got abducted, Cap'n Cuttlefish decided to recruit a random brave Inkling as the latest member of his New Squidbeak Splatoon, and acts as their guide. Cuttlefish dubs this new member as Agent 3, because before the Zapfish abduction, Cuttlefish already recruited Agents 1 and 2, which are in fact his granddaughters, Callie and Marie.
Cap'n Cuttlefish and the leader of the Octarians, DJ Octavio, actually were on friendly terms before the Great Turf War. Cuttlefish also appears to be friends with Sheldon's grandfather, who was a weapons expert.
Despite his elderly appearance, Cap'n Cuttlefish apparently finds himself very handsome. He also has a favor for sea biscuit. He also seems to be fond of rapping as of Splatoon 2, and has become friends with Pearl of Off the Hook due to their shared interest during the events of the Octo Expansion.