One consistent trait that the best anime always seem to have is a wide array of characters that leave an impact on the viewer. The recent breakout star, Demon Slayer: Kimetsu no Yaiba is no different in this regard, having a lot of its own unique and colorful characters making up the cast.
Given their numbers, it's not unlikely for some of these characters to have paths that rarely cross, if ever, or even never meet at all. Even so, it'd be nice to and while there may be some in Demon Slayer's cast that'll never team-up, there's many that would be amazing if they did.
10 NEED TO SEE: Inosuke & Zenitsu
Considering that their first meeting involved Inosuke beating the ever-loving snot out of Zenitsu over the box that houses Nezuko, it's obvious that Inosuke and Zenitsu got off to a bad start. So much so that in Demon Slayer's first season, the tandem of Tanjiro and Inosuke was prioritized over that of Zenitsu and Inosuke. That's not to say that the latter is impossible, though.
Despite their personalities being polar opposites, it would be a nice change of pace to see the two get along for once and fight together rather than being split up... or just seeing Zenitsu cowering behind somebody again.
9 WE'LL NEVER SEE: Yushiro & Tengen
Yushiro is an assistant to the demon doctor Tamayo and holds her to the highest of esteems. So much so that he typically doesn't get along with anyone else; much less the Demon Slayers. The Hashira are especially pretty hostile towards all demons, so it's unlikely that Yushiro would ever team up with any of them, but especially not Tengen.
The Sound Hashira is a rather cold and callous towards people normally, his attitude towards demons is much worse too. Considering his rough personality, Yushiro would probably avoid Tengen just as he would any of the other Hashira, if not more so.
8 NEED TO SEE: Tanjiro & Tomioka
It's strange that Tomioka - who was the one who convinced Tanjiro to become a Demon Slayer in the first place - and Tanjiro never join forces back in Season 1 of Demon Slayer. Sure, Tomioka rescued Tanjiro and Nezuko a few times and he's clearly on their side unlike some of the other Hashira, but Tanjiro never actually fought with him.
Seeing the two fight together might actually be a good way of showing just how far Tanjiro has come since their first clash way back, as well as some potentially amazing combo moves since they both use the style of Water Breathing techniques.
7 WE'LL NEVER SEE: Urokodaki & Kuwajima
Believe it or not, both Sakonji Urokodaki and Jigoro Kuwajima, the mentors who trained Tanjiro and Zenitsu respectively were both former Hashira. As such, there must have been times where the two had teamed-up on missions. Sadly, it's unlikely that those moments or future ones will ever play out.
While it would be something to see these two masters team up just like their pupils do, they are both rather past their prime and have mostly retired to train their students to be great Demon Slayers. It seems like seeing these two fight together (or even solo, for that matter) just isn't in the cards.
6 NEED TO SEE: Nezuko & Zenitsu
Zenitsu is a bit of a coward, that much is obvious, but one of his other well-known traits is his love for any girl he thinks is cute. Nezuko's no different in this regard, despite being a demon Zenitsu took a shine to her the moment he saw her. While she doesn't seem to reciprocate any feelings for him, though still, it could be interesting to see the two work together.
Sure, Zenitsu doesn't look too impressive on his own, but when push comes to shove he has proven himself before and Nezuko herself is pretty good in a fight too. Considering how willing Zenitsu was willing to swallow his fears for her sake, seeing a team-up between the two could better than anyone would think at first.
5 WE'LL NEVER SEE: Enmu & Wakuraba
Both Enmu and Wakuraba are demons who are part of the 12 Kizuki, a group of powerful demons that work for the Demon Lord Muzan Kibutsuji. Both being part of the "Lower Moons" half of the group, the two have wildly different personalities that make it hard to see them actually working together.
Among the two, Wakuraba was more loyal to Kibutsuji, respecting him above all else, while Enmu is more careful and intelligent, always wary of their leader and generally not caring for his fellow members of the 12 Kizuki. These personalities just don't mix well. What's more, given the fate of the Lower Moons, there's actually zero chance now than there's ever been that the two would ever team-up.
4 NEED TO SEE: Shinobu & Kanao
The family that fights together, stays together. Despite not being related by blood, Shinobu and Kanao are adoptive siblings and, as such, are very close. Kanao's also designated as a "Tsugoku" to Shinobu, meaning that she is her apprentice that is destined to take her place as a Hashira. Though before any of that has a chance to happen, it would be something to see a true team-up between the two.
Sure, they have worked together before during the Natagumo Mountain Arc, but we didn't see them fight side by side during that. As someone close to Shinobu, a high ranking Hashira, Kanao is bound to have some techniques that pair well with Shinobu's.
3 WE'LL NEVER SEE: Sanemi & Nezuko
Out of all the Hashira, Sanemi Shinazugawa is the most aggressive among them. He has zero patience for demons, given his own history and his many scars, it's not exactly hard to see why he has such an abrasive attitude. With that "no tolerance policy," he was one of the many in the Hashira who saw Nezuko as a monster and was the tried to prove it too.
Given that as their first meeting, the feeling of dislike is probably more than just a little mutual, thus making the likelihood that Sanemi and Nezuko would ever team-up for any reason about slim to none.
2 NEED TO SEE: Kagaya & Tamayo
Kagaya Ubuyashiki is the 97th leader of the Demon Slayer Corps, a title that's not given to just anyone. At the end of Tanjiro's trial, he mentions Tamayo for a brief moment, implying that the two are actually familiar with one another. Perhaps this means there's a chance the two could team-up, combining their incredible skillsets to stop a shared enemy.
Given that they're both out to put a stop to Kibutsuji, it would make a lot of sense for them to join forces. After all, they say "the enemy of your enemy is your friend."
1 WE'LL NEVER SEE: Tanjiro & Kibutsuji
There are times when the hero and villain have to work together to stop a common threat. In the context for Tanjiro and Kibutsuji, though, there is simply never going to be a time that would see them teaming-up.
Kibutsuji kills almost every single member of Tanjiro's family and turns the only surviving member, Nezuko, into a demon at the start of the story. From there, Tanjiro's sole focus has been to hunt down Kibutsuji and his collaborators for vengeance and to turn his sister back into a human. Their goals would just never line up, making this scenario next to impossible.