Comic book character, owned and published by Marvel Comics. He was created by Kelly Thompson and Daniele di Nicuolo. He made his first appearance in 2018's West Coast Avengers #6, but got his formal introduction and name in the next issue.
Jeff is a baby landshark. He has four legs, he can get along fine on land or underwater, he's got sharp, intimidating teeth, and he's very, very cute.
In the first issue of the 2018 edition of "West Coast Avengers," the giant-headed, tiny-armed supervillain M.O.D.O.K. unleashed a horde of landsharks on Santa Monica, California. When the L.A.-based superteam foiled the attack, one of the sharks, a very round and adorable baby, was adopted by Gwenpool. Jeff's ownership has bounced from Gwenpool to Deadpool to Kate Bishop, though he's generally been a supporting character, at least as far as superhero action goes.
In September 2021, Jeff got his own series: "It's Jeff!" It was written by Thompson and illustrated by the Japanese art team Gurihiru. It appeared as part of Marvel's Infinity Comics program -- digital comics in a vertical scrolling format for Marvel Unlimited subscribers.
"It's Jeff!" was mostly dialogue-free and followed Jeff roaming the city getting into and out of trouble and causing humorous problems for Marvel's superheroes -- stealing Captain America's shield to go sledding, throwing superheroes into a panic by swimming shark fin up at a pool party, eating everyone's Thanksgiving dinner, and whatnot.
Jeff doesn't have many powers. He has a gigantic appetite (which gets even bigger when he's briefly bonded to a Venom symbiote during the "Venom War" crossover storyline), he's got a tremendous shark bite, and he's adorable and everyone loves him.
Jeff was already a decently popular character, but he got a big boost when he appeared as a playable character in the "Marvel Rivals" computer game. Jeff is a Strategist character, which means he's able to heal or buff his allies. He has a water blast attack to damage opposing players, and he uses water and bubbles to heal players on his team. His Ultimate attack lets him swallow other characters, both allies and enemies -- allies get healed, enemies get damaged, and if Jeff can spit out enemies over a ledge, it can lead to a total party kill.
Jeff is still primarily a humor character, and in superhero comics, the future viability of humor characters depends on their popularity with readers, the cleverness and skill of writers and artists, and the fickle patience of editors and publishers. But Jeff is still lots of fun, and hopefully, he'll stay fun for a nice long time.