Shermy is a small, light-gray cat. Shermy has a long tail, a proportionally large mouth, small black eyes, gray freckles, and large ears. The extremes of his left fore leg and right hind leg are white instead of light-gray. Shermy wears a light-red shirt and a khaki satchel.
Shermy is adventurous and outgoing, and is curious about Ooo's extensive past. He apparently hates the library. It should be noted that Shermy's personality greatly resembles Finn's during the first few seasons.
Shermy is Finn's possible reincarnation, considering their similarities.
Beth is Shermy's close friend, not much is known about their friendship as it is not shown in the show.
By far, nothing is known about Shermy's past, save that he came to live with Beth, he is fully aware of her lineage as the Pup Princess.
Somewhere in the relative past they began living in Marceline's now-abandoned house. In "Graybles 1000+", he or Beth apparently mastered the ability to play Marceline's banjolele with enough proficiency to sound like her.
While Finn was driven by a primal need to help, Shermy is defined by his unbridled curiosity, roaming Ooo to collect its "treasure", mostly a hoard of garbage and relics of Finn's era. In "Come Along With Me," his failed attempt to capture a Prize Ball Guardian for its loot ends in him acquiring Finn's mechanical arm.
Curious of its origins, but unwilling to visit the library, he accepts Beth's idea to seek the fabled "King of Ooo". This is revealed to be BMO, who now occupies a house on the top of Mount Cragdor, surrounded by relics of his past, and thought to be merely a legend. There Shermy and Beth start accidentally destroying parts of BMO's collection with Shermy wearing Simon Petrikov's glasses and Marceline's anti-smoking T-shirt, which prompts BMO to almost politely throw them out.
However, when Beth shows BMO the arm, he finally consents to tell Shermy about the Gum War, the battle against GOLB and the final fate of the people of Finn's era, albeit misnaming Finn as "Phil." He then politely throws them out.
Shermy then remembers to have seen Fern's tree (previously shown in "Lemonhope Part 2" and "Graybles 1000+") in his travels around Ooo, later climbing its top, and finding the restored Finn Sword. He promptly takes the sword for himself, and poses with it like Finn during the opening of the show, symbolically accepting his mantle as the new Hero of Ooo.
- Shermy was based on Steve Wolfhard's cat Shortcut.
- Shermy's name was based on Jeremy Shada's first and last name.
- There is a possibility that Shermy is one of Finn's future reincarnations.
- Shermy's personality is similar to that of Finn's in earlier seasons.
- Shermy is seen holding up the Finn Sword in the same pose Finn had at the end of the show's opening sequence.
- Shermy's red shirt is the opposite of Finn's blue shirt.
- Additionally, counting Shoko, whose shirt was green, every one of them wore a shirt based on one of the three additive primary colors.
- Shermy carries a satchel, just like Finn carries a backpack.
- Shermy's head bears a slight resemblance to Finn's hat.
- Shermy eats cookies in the shape of his head, just like the Finn Cakes.
- Further suggesting the connection between Shermy and Finn, it was decided that Shermy would be a cat because of Finn's line in "Mortal Recoil," where he says, "I'm a cat, I'm an agile cat!"