Demon Slayer's Hashira might be the strongest demon slayers in the series, given how they are considered the masters of their works. This could mean that they would be better off without any assistance. Other than the assistance from Kusagai Crows (in locating demons), some help from supernatural creatures might make these demon slayers overwhelming individuals.
So, the most logical supernatural beings in this case would be pocket monsters from Pokémon. However, similar to the issue with the Kusagai Crows, the pocket monsters work well with users with similar personalities. This article will compile 9 Pokémon that would be perfect partners for Demon Slayer's Hashira.
Disclaimer: This article contains potential spoilers from the Demon Slayer manga series and has the author's opinion.
Lucario, Mimikyu, and 7 other Pokémon who'd be the perfect partner for each of Demon Slayer's Hashira
1) Shinobu Kocho - Dustox

The Insect Hashira from Demon Slayer and quite possibly the most famous one from this group of demon slayers, Shinobu, is a poison user who experiments with deadly material, hoping to create a lethal drug.
To help in this, Dustox might be her match made in heaven, as the Pokémon is a Bug+Poison type. Dustox might provide Shinobu with toxic elements that would make a perfect poison to instantly kill demons and fulfill some other purposes of the Hashira.
2) Kyojuro Rengoku - Blaziken

The smile of Demon Slayer's Hashira and the chivalrous one, Rengoku, was one of the strongest Hashira who lost his life against Akaza. If he had the assistance of a supernatural being at that point, it might still be with Tanjiro and his group.
Even though Rengoku was known for his flames, a Pokémon with both flame and fire might fit him, and that would be Blaziken. A Fire+Fire type Pokémon known for its ability to fight its opponents (just like the Hashira) seriously. Moreover, Blaziken's fire might amplify Rengoku's attacks, making them a perfect match.
3) Giyu Tomioka - Gyarados

Tomioka, the master of Water Breathing, is one of the strongest demon slayers from Demon Slayer. Moreover, an individual that might perfectly complement him would be a water-type Pokémon, and what better choice in this case other than Gyarados?
Gyarados is a water+flying type Pokémon known for its size and water-based attacks. The Pokémon might also amplify Tomioka's Water Breathing in battle, giving it a realistic and overpowered touch.
4) Tengen Uzui - Lucario

More proudly referred to as the God of Festivals, Tengen Uzui is the Demon Slayer's Sound Hashira. Moreover, a key element of his personality is his prideful nature, which would make a prideful Pokémon his perfect partner.
Out of the hundreds of Pokémons, Lucario might be the perfect match for the Sound Hashira. As a Fighting+Steel Pokémon, Lucario is also known for his prideful display on the battlefield. The similarity in these characters' personalities might give Uzui a boost in any fight.
5) Mitsuri Kanroji - Mimikyu

Mitsuri is the most playful and friendly Hashira from Demon Slayer. However, she is also conscious about her looks due to her past encounter where she was rejected due to her natural hair color. So, to accompany her in battle, a Pokémon with a similar personality would not only fit her but make Mitsuri comfortable too.
Mimikyu is one of the biggest mysteries of Pokémon because it has yet to reveal its true appearance. Being a ghost+fairy Pokémon, it is expected to be scary, but Mimikyu is one of the friendliest creatures who is only conscious about his appearance, making it the perfect partner for the Love Hashira.
6) Muichirou Tokito - Froslass

The Mist Hashira and a master of Mist Breathing, Muichirou is one of the most powerful Hashiras. The key to the character's personality is his quiet breathing technique that could take down the strongest of demons.
Froslass is an ice+ghost Pokémon known to inhabit snowy areas. Moreover, the Pokémon's ice-based power make it reminiscent of Muichirou's breathing technique, making these two the perfect match in battle. Froslass' attacks might also amplify Muichirou's breathing technique due to their similar nature.
7) Obanai Iguro - Serperior

Obanai Iguro is the Serpent Hashira who invented his own breathing technique, the Serpent Breathing. This technique gives him the ability to imitate a serpent's movement and kill various demons in a single thrust. To compliment him in battle, a Pokémon with similar abilities would prove beneficial.
Serperior is a grass Pokémon with a snake-like body that might be the perfect partner for this Hashira. Serperior's ability to stop enemies with just a glare might help Iguro use his breathing technique against quick enemies.
8) Sanemi Shinazugawa - Staraptor

Sanemi, the most violent Hashira and the master of Wind Breathing is one of the strongest Hashira. Known for his barbaric behavior with his fellows, Sanemi's trademark is to go into battle alone. So, a Pokémon with a similar fighting style might increase Sanemi's chances of winning on the battlefield.
Out of all of the rowdy Pokémons, Staraptor might be the perfect fit for the Wind Hashira. Just like Sanemi, Staraptors leave their home at an early stage. Moreover, both of them are violent and a menace in a fight.
9) Gyomei Himejima - Golem

Quite possibly the strongest demon slayer in the series, Gyomei Himejima is the master of Stone Breathing. Known for his extraordinary raw strength, Hashira's aura speaks of his expertise in battle. To accompany, a Pokémon with similar powers would fit perfectly.
Out of every stone Pokémon, Golem might be a perfect fit for the Stone Hashira due to its raw strength. As a rock+ground Pokémon, Golem might imitate Gyomei's Stone Breathing down to the very detail, thus enhancing the Hashira's powers.
Final thoughts
Pokémons might actually be the perfect partners for Hashira against demons as these supernatural creatures could look into the demon slayers' blindspots, thus helping them kill as many demons as possible. Moreover, these supernatural creatures might also help these Hashira take an Upper Moon Demon without any worries.
Related Links
- 10 unsolved Demon-Slayer mysteries from the Hashira training arc
- 10 Demon-Slayer characters who'd be perfect Hunters in Solo Leveling
- 10 most powerful Demon-Slayer characters, ranked
- 10 Demon Slayer characters who lost their families
- 10 Demon Slayer characters who deserve Observation Haki