Demon Slayer's third season has come to a close and two more upper moon demons have been slayed. During this epic battle, fans were given further insight into the powerful abilities select superior demon slayers possess. The love Hashira Mitsuri Kanroji, and the mist Hashira Muichiro Tokito, both showcased their marks.
When a mark is produced, the demon slayer receives enhanced power, stamina, and strength with breathing techniques. It is unknown if all Hashira and lower ranking demon slayers have access to this ability, however. Kyojuro Rengoku never produced a mark, not even during his jaw-dropping battle against the Upper Three demon Akaza at the end of the Mugen Train arc. The recent manifestation of Kanroji and Muichiro's marks have fans questioning whether Rengoku should have gotten his mark during his fight with Akaza. Further, fans wonder whether a mark would have made a difference in how the fight ended.
Rengoku Qualifies For A Mark
The first time a mark appeared was toward the end of the Entertainment District arc when Tanjiro's scar transformed as he gained the strength to behead the Upper Six demon Gyutaro. The mark appeared due to the stress of the situation, as well as Tanjiro's determination to slay the upper rank and end his reign of terror. A similar set of circumstances also pushed Muichiro into producing his mark, as his mark appeared when someone else's life was in danger. This seems to imply that skilled demon slayers should be able to produce a mark in high stress situations. Yet, Rengoku's fight to the death, in which he was protecting his fellow demon slayers, did not result in him awakening a mark of his own.
The argument could be made that only those with a sufficient skill level and latent power can produce a mark. However, Rengoku showcased incredible strength and skill, even Akaza acknowledged Rengoku's fighting prowess. The only thing that held him back was his vulnerable human body. Even without the ability to regenerate, Rengoku nearly ended the Upper Three demon for good, all without a mark. Rengoku maintained just as much strength as his Hashira peers, if not more, so there's currently no reason why he didn't produce a mark.
Rengoku Would Have Been OP
Kyojuro was descended from a long line of flame breathers, the breathing technique most closely related to sun breathing. His strength and disciplined training indicated that he deserved to get his mark. If he had produced one during the fight with Akaza, considering how close the fight was in the end, there's no doubt he would have slayed the Upper Three demon. However, despite fighting to protect Tanjiro, Zenitsu, Inosuke, and Nezuko, as well as every other passenger on the train, Rengoku never reached that next level.
Recent fan discussions debate whether the Upper Five and Upper Four demons deserved their high ranking considering they were fairly easy to defeat. This means that Rengoku battled one of the most powerful demons in the upper rank system. Had he awakened a mark, Rengoku would have undoubtedly unlocked a new tier in the Hashira ranking system – he had the potential to be the most overpowered Hashira in the series. Nevertheless, his sacrifice will undoubtedly continue to shape the heart of the story to come.