Overqwil is a large, spherical piscine Pokémon that resides in the sea. All over its body are overgrown conical spikes whose colors correspond to those on Overqwil's body. Its top half is a black color while its bottom half is light yellow; the two halves meet in the middle and form a wave pattern. The spikes on the top half of its body have parts of purple at their tips, and there are two very long spikes at the top and bottom of its body that jut out in a straight direction. Above its eyes and around part of the top half of its body, Overqwil has circular purple dots. Its lips also have a purple coloration and appear puffy, and its small light-yellow eyes have purple pupils and are always fixated in an annoyed expression. In between its eyes are two pairs of spikes, the outer ones being longer and resembling eyebrows. Its large tail appears flat with a black outline and a purple inside, which also has a pattern that resembles the letter "Q." Overqwil has a very aggressive personality, which has given it the nickname "sea fiend." Its diet consists of poison.
Overqwil and its pre-evolved form, Hisuian Qwilfish, are the only known Pokémon capable of learning the move Barb Barrage.
Overqwil evolves from Hisuian Qwilfish in Hisui.
(For specifics on this Pokémon's evolution in the games, refer to Game data→Evolution data.)
Overqwil appears to be based on pufferfish or porcupinefish, which are covered in spines and are poisonous to eat. However, Overqwil's oversized spikes along with their ability to inject poison may be inspired by other venomous marine animals such as lionfish or sea urchins. Overqwil and its pre-evolution's notoriety amongst fishermen may be a reference to the significant threat that lionfish pose as an invasive species in the Atlantic Ocean.
Overqwil's Dark-type might be based on the illegalisation of the fugu dish, due to its toxicity, and the following illegal preparation and consumption of said dish. The law was put in place a few times in Japanese history (and eventually repealed each time), including the 1800's, when Pokémon Legends: Arceus appears to take place. The illegal fugu market continued and thrived for so long that the governing shogunate became incapable of enforcing the law, which might have inspired Hisuian Qwilfish being troublesome for fishermen, and could also be a parallel to the lionfish's invasive status, as neither the dish, the lionfish, nor the Pokémon could be controlled. As Hisuian Qwilfish and Overqwil appear to no longer be present in the current era of the Pokémon world, they may be extinct, which could then be another reference to some porcupinefish being endangered or threatened due to overfishing for the fugu dish.
Overqwil's physical appearance along with its signature move, Barb Barrage, together are reminiscent of naval mines. It also resembles a morning star or the spiked end of a flail. Based on its Japanese category, it may also be partly inspired by the kenzan (剣山, lit. "sword mountain"), a device consisting of a plate with numerous needles or spikes on it, used to fix flowers in place for the art of ikebana (flower arranging).
The marking on Overqwil tail looks like the uppercase "Q" letter. On its preevolved form, Hisuian Qwilfish, the same marking resembles the lowercase "q". These may stand for their English names.
Overqwil may be a combination of over or overkill, quill, and Qwilfish.
Haryman may be derived from ハリーセン Harysen (Qwilfish) or 針千本 harisenbon (porcupinefish), with the 千 sen (thousand) part replaced with 万 man (ten thousand or myriad), continuing the numerical theming.