Qwilfish is a sea dwellingpiscinePokémon with a spherical body and a flat, paddle-shaped tail. The upper half of its body is dark teal and the lower half is light greenish-yellow. Its tail is tall and thin with a dark teal outline, light greenish-yellow inside, and two dark teal dots in the center that resemble the Japanese character ハ Ha. Its eyes are even with its body and are always seen slanted angrily. Below its eyes are its puffy pink lips. All around its body are sharp, conical spikes, which evolved from scales over time.
In order to attack, Qwilfish drinks over 2.6 gallons (10 litres) of water to inflate its body. It then uses the water pressure to shoot poisonous quills at its foe, which can cause fainting. Qwilfish is briefly left defenseless shortly after firing its quills; this is believed by some expert fishers to be the opportune moment to catch this Pokémon. Due to its round body, Qwilfish is not a great swimmer, and when filled with water it is even harder for it to swim. When Qwilfish faces a larger opponent, it will inflate its body to match its size as close as it can. If it is startled while inflated, it will spit out all its water, propelling itself away.
In Hisui, its upper half is black, while its lower half is pale yellow. Its body is covered in spikes; the spikes on its upper body have dark purple tips, except for the two spikes on its forehead, which are also smaller in size. Hisuian Qwilfish can spray poison from these spines. There are crescent purple lines underneath Hisuian Qwilfish's eyes, and its lips are also colored purple. Its flat tail has a black outline with a light yellow inside, and a black pattern that resembles the lowercase letter "q." Hisuian Qwilfish is despised by the fisherman of the region, due to its poison wreaking havoc in the seas it lives in.
A swarm of Qwilfish appeared in Querulous Qwilfish, where they were angered by pollution that occurred as a result of an accident that occurred during the construction of the Battle Tower. Crystal captured all of them with her skills. Afterwards, she found a baby Qwilfish stuck in the toxic waste; as she prepared to take it to a Pokémon Center, Suicune appeared, healing Qwilfish and purifying the seawater. It is unknown if the Qwilfish are still under Crystal's ownership or were released afterwards.
Qwilfish seems to be based on pufferfish or porcupinefish, which inflate their bodies when threatened and are covered in spines. Many species are incredibly toxic when ingested, which explains Qwilfish's Poison type. However, its ability to directly inject venom through its spines is similar to a lionfish. It is also reminiscent of a naval mine due to its round body and ability to explode.
The horn-like spikes on Qwilfish's forehead along with its general body shape may also be a reference to boxfish and cowfish species like the longhorn cowfish. When stressed, cowfish can exude a deadly toxin in the mucous secretions of their skin, similar to how Hisuian Qwilfish is known for spraying poison and getting it everywhere.
Hisuian Qwilfish'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: Arceusappears 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 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.
The marking on Hisuian Qwilfish's tail looks like the lowercase "q" letter. On its evolved form, Overqwil, the same marking turns into the uppercase "Q". These may stand for their English names.
Qwilfish may be a combination of quill and fish.
Harysen may be derived from 針千本 harisenbon (porcupinefish). The character 千 sen in 針千本 harisenbon means thousand, and this numerical theming is continued with Overqwil's Japanese name.