Drop down the information below with caution unless you have completed Hero Mode or Octo Expansion. Spoilers are below.
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In Splatoon 2, he joins the New Squidbeak Splatoon to assist Marie and Agent 4 in rescuing her cousin Callie from DJ Octavio's control, personally driving his truck, with the Squid Sisters' logo on the sides and a stage on top of the trailer for Marie, into the stadium where Agent 4 was battling Octavio and Callie, allowing Marie to shoot Octavio's hypnoshades off Callie and attempt to override Bomb Rush Blush with Tide Goes Out, until she begins playing Calamari Inkantation, which breaks Callie completely free of Octavio's spell, and she rejoins her cousin on Sheldon's truck to perform the song together once more.
THE SQUID SISTERS ARE BACK!
Sheldon then arms Agent 4 with a modded Rainmaker to take down Octavio and his Octobot King II once and for all. Octavio attempts to escape, but Agent 4 damages the Octobot King II enough that it malfunctions and crashes back into the arena, leaving Octavio vulnerable for Agent 4 to hit directly with the Rainmaker, resulting in the Octobot King II swelling up like a balloon before exploding into tons of Agent 4's ink, resulting in Octavio's capture and imprisonment once more. Sheldon then returns to Inkopolis to resume running Ammo Knights.
Spoilers end here.
Personality and traits
Sheldon is a genuine military fanatic and is fascinated by his weapons. He has developed nearsightedness due to staying up late every night messing around with small parts.
Though he is a horeshoe crab, Sheldon wears his whole shell on his head, like a hat. He also wears a pair of goggles, a khaki scout uniform, and a red scarf around his neck. He has buck teeth and pale yellow skin.
Sheldon's name comes from the word "shell," as he is a horseshoe crab which is a species known to have shells.
Sheldon's grandfather, Ammoses Shellendorf, has blueprints that are aquired from beating each boss in Hero Mode. There are five blueprints total, which include the blueprints for the Hero Shot Replica.
There also are three more blueprints obtained after completing the DJ Octavio challenges of the amiibos.
Ammoses Shellendorf was the most likely individual who stood behind Cap'n Cuttlefish in a photograph of them during the Great Turf Wars.