#ipponmatsu

Shusui was a black sword that belonged to legendary samurai, Ryuma. In Thriller Bark it was wielded by Ryuma’s zombie (with Brook’s shadow) and once Zoro saw the blade, he wanted it for himself, to replace destroyed Yubashiri and was willing to take it from the enemy's dead body. Though the fight was relatively short, Zoro’s skills were acknowledged by Ryuma and in result, Shusui was given to pirate.

Zoro honored his opponent by taking sword and being willing to pretend the match never happened.

Similar to Sandai Kitetsu, Zoro both chose a sword and was chosen (accepted) by the blade. But unlike Kitetsu, Shusui, as the most precious treasure of Wano country, couldn’t truly belong to him. The katana served its master well and Roronoa appreciated it very much, but since Dressrosa arc, he was constantly nagged by people of Wano to give it back. He didn’t care for their complaints and did not plan to do so, yet once asked by Kozuki Hiyori to return it to its rightful place, Zoro finally agreed. Because in the end he respected the dead swordmaster and how much the black sword meant to people of Wano. It was a sacred symbol, a heirloom of legendary samurai Ryuma while he was an outsider. Although Zoro proved his worth to Ryuma zombie, the sword (and Ryuma’s body) was stolen in the first place and Hiyori was willing to give up memento of her late father (personal treasure) for Shusui (a national relic). Such sacrifice proved how much Ryuma’s sword meant to people of Wano and Zoro, despite his claim, returned the black blade to where it belonged - asking only to let him visit Ryuma’s grave once the battle is over.

Shusui was another heirloom, a sword burdened by past and its meaning. It served Zoro well, but it couldn’t truly belong to him.

For returning Wano’s national treasure, Hiyori promised Zoro no less great katana - ENMA.

A legendary sword given to Hiyori by her late father, Kozuki Oden that is said to be the only blade that even injured Kaido. A wonderful gift, but the same as previous swords, it is someone else's heirloom passed to Zoro. Another memento of a dead person. Another burden to carry on. 

While Zoro trained with his new sword, Tenguyama Hitetsu (the creator of Sandai Kitetsu) told him the story behind Enma - Oden’s sword and Kuina’s Wado were birthed by the same man, Shimotsuki Kozaburo. Who, ironically, is Kuina’s grandfather according to SBS. What Zoro may have known now, since he did met Kozaburo as a child.

Anyway, Hitetsu assumed that Hiyori perhaps recognized Wado and because of that offered Zoro her heirloom as a replacement for Shusui. I personally think the reason was more selfish than that. Once Zoro saved her, Hiyori said there was a prophecy that “in time of the resurgence of the Kozuki line, strong, kind samurai from across the seas would come to our aid” and Zoro fits perfectly. Even more, since Zoro’s hometown, the Shimotsuki Village is in fact strongly connected to Wano by Kozaburo no less.

Hiyori, as Oden’s daughter, wants Wano to be a free, safe country again but for this to happen, Kaido must be defeated first. Zoro already proved to be a reliable ally. He came To Wano with her missing brother and Kinemon. He saved her and Toko from Orochi’s assassin and later, from enemy attack once the fight broke out after Yasuie’s death. He is strong (something she saw for herself) and to some degree has the samurai feeling about himself. 

Above everything else, Zoro will fight in the upcoming war against Kaido for the Kozuki clan even though he (like all Straw Hats) has no real obligation to her family. Whether she believed in prophecy or not, by passing to Zoro her family heirloom, Hiyori burdened him with her hopes and dreams for victory and justice for her murdered parents and for Wano country as a whole. She gave an outsider Oden's precious sword that can injure Kaido and by that, placed faith in Zoro. She can’t use the sword, she can’t fight in battle, but Zoro will carry on Oden’s (and her) wish to protect Wano into battle. 

Was Zoro aware of such a possible burden? Who knows. Still, he accepted the blade despite its dangerous nature - and any eventual burden (someone else’s unfilled dreams and hopes) to carry on.

Frankly, Enma and Sandai Kitetsu have a lot in common. Both are dangerous blades that drive weak swordsmen into madness and/or ultimately lead to their death. Both need to be tamed. And Zoro likes them very much, despite warnings of other people. Even manga presents their “introduction” as accepted and appreciated swords by Roronoa in similar way:

Yet there are two major differences between Kitetsu and Enma, what also are true for the three previous swords too.

First, already mentioned and explained the matter of heirloom and connected to that “burden” Zoro carries on alongside. In the case of Enma, once the battle is over and Wano saved, all the faith and hopes put in Zoro by Kozuki Hiyori through the Enma should not burden him anymore. Yet since Hiyori entrusted him the memento of her father, Roronoa will need to make sure to never bring shame to her (and by extension, the Kozuki clan) in later adventures & fights he will face one day. But frankly, this is the same with all of his swords bestowed to him by people who were important to him or which he shared mutual respect. It is strictly connected at core to sword fighting philosophy; to be worth of great blade.

Secondly, Wado Ichimonji, Yubashiri, Shusui and Enma, all those swords are desired and appreciated by swordsmen all over the world while Kitetsu, as a cursed blade, was not wanted. Kitetsu was feared by people, put in barrels between low quality swords in hopes to get rid of it. Even sword maniac like Tashigi, once learning the truth about the katana, did not want that blade despite its high grade and low price on it. She was even sorry for insisting previously Zoro should take it. Kitetsu drove previous owners into horrible deaths to the point “you will not find a swordsmen who’d dare to use a Kitetsu these days”:

Zoro felt that the sword was cursed, before Ipponmatsu had a chance to explain why Kitetsu had such a low price on it. And after learning the horrible truth, he decided to take the blade. The unwanted, bloodthirsty, cursed blade that brings madness and death because no one else would dare to use it. Zoro liked the challenge. Oh, he liked it so much he decided to test his luck (fate) against the curse of Kitetsu, putting his swordmaster future at stake.

The same as Zoro chose Kitetsu, the sword chose him. Like Ipponmatsu said, the sword chooses its owner. And yes, Zoro wanted to have Wado and Shusui but these blades were heirlooms burdened with someone’s dreams and greatness while Kitetsu was the unwanted, dangerous blade with a horrible past that Zoro faced only with his own strong will. No hidden promises to carry on, no ambitions or honor or hopes passed on him by previous owners. No legendary status to live up. Like Wado. Like Shusui. Like Yubashiri. Like Enma.

Sandai Kitetsu is this wild, bloodthirsty sword that was challenged by Zoro and decided to spare this insane man who looked at it, felt the cursed nature and did not throw it away but appreciated it. Something that, I imagine, didn’t happen in years. And there is very strong bond between Roronoa and Kitetsu, because Zoro always can tell apart the cursed blade from other swords

and he kept Kitetsu usually in left (dominant) hand during fight. All those little details give the impression Kitetsu is much more spiritually connected to Zoro’s core than Wado.