Why didn't Rengoku unlock his Demon Slayer Mark? Pretty sure that he met all the requirements. He was in a life-threatening situation. He uses Flame Breathing, so his temperature already increases by using it.
Kyojuro Rengoku was the first Hashira to die at the hands of an Upper Moon demon, due to an unfortunate encounter with Akaza. Despite the intensity of his battle against the demon, he never manifested his mark, and undoubtedly did not even know the conditions for doing so.
The rankings of demons start off from just a normal demon to the Twelve Kizuki, who have six lower and upper ranks. But, the reason Rengoku didn't get his mark yet is still being investigated. Most people say that Rengoku could have gotten his mark if Tanjiro awakened his mark near Rengoku.
He almost died if Tanjiro didn't pop in. But Rengoku did an amazing job trying to stop Akaza with his injuries. Even tho he died, he still made injuries that Akaza couldn't easily heal. If Rengoku awakened his mark, he could've killed Akaza without even dying.
Rengoku was able to hold off an Upper Moon 3 Akaza and even managed to get him desperate towards the end. Every aspect of Rengoku's combat abilities seem to be better than Tengen's. However, he would put up a good fight as it will not be an easy victory for Rengoku.
After awakening his demon slayer mark, Giyu greatly overpowers Akaza. However, he is able to keep up with the fight only because of Tanjiro. If Giyu had soloed against Akaza, he would have died.
When it comes to pure strength, Shinobu Kocho is the weakest Hashira in Demon Slayer. She doesn't even have enough strength to decapitate a demon's head and she makes up for it by injecting poison into the demons.
Shinobu Kocho Is the Weakest Hashira
Of the three Hashira who were killed by Upper-Ranks -- Rengoku, Shinobu and Tokito -- Shinobu was the only one who was killed almost instantly when she encountered Doma, Upper-Rank Two.
Gyomei Himejima – Stone Breathing Style
Gyomei Himejima is the strongest Hashira in the Demon Slayer Corps.
In the anime Demon Slayer, Tanjiro Kamado possesses a large scar on his forehead that, at the beginning of the series, just seems like a birthmark or burn scar. It isn't until later that we the audience discover that it is far more than just a scar. The mark on Tanjiro's head is actually a Demon Slayer Mark.
Rengoku's Unwavering Will Was His Greatest Weakness
Since he believes it is his responsibility as someone strong to protect those weaker than him, he will make sure he does just that. Rengoku sees any mission through to the very end, even at the cost of his life.
Rengoku's performance against the demons on the Mugen train in the Demon Slayer movie was impressive enough to draw admiration from even Inosuke, and Rengoku ensured that not one of the 200 passengers died. Unfortunately, Rengoku succumbed to Akaza, the Upper Rank Three demon, after initially almost defeating him.
Yoriichi lived till old age, likely due to him having been born with the mark and thereby avoiding the curse altogether. Gyomei on the other hand did not activate his Mark until he was already older than 25.
Rengoku is a limited-time Eye Bloodline with a rarity of 1/200. Additionally, it can be obtained from the Sen Exams Dungeon in the Dungeon Game Mode with a rarity of 1/20.
I know this is 2021 guys but giyu is stronger than rengoku. The thing is, the style between the flame and the water breathing is totally different.
Human Rengoku managed to keep up with Akaza, even if he was playing, Akaza complimented his strength. Considering his base strength, If he became a demon he'd definitely rise to the Kizuku, likely taking the spot of upper 3-4.
1. Gyomei Himejima. Gyomei Himejima, the Stone Hashira, is the strongest Hashira in the Demon Slayer corps in terms of strength and swordsmanship. He is the only person who uses spiked flails and an axe instead of standard swords to kill demons.
Sanemi's blood is known to be enticing yet intoxicating to Demons, which he used to bait and stab Nezuko. Sanemi is clearly the least popular Hashira among Demon Slayer fans.
As a Hashira, Tengen possesses incredible strength, speed and agility which allows him to fight at super-human levels, as he is the sound Hashira, he also possesses incredible hearing abilities and uses sound waves to locate his enemies and to listen and beware of his surroundings.
But, when it comes to who was the better Hashira, Tengen wins by a considerable margin. His experiences as a shinobi and as a Demon Slayer far exceeded Rengoku's, and led to him developing far superior strength, battle tactics, and techniques.
1) Shinobu Kocho
The Insect Pillar of Demon Slayer Corps, Shinobu is undeniably the smartest individual. She might not be physically stronger than others, but her intelligence is what kept her going.
The point is, without Selfess State, Gyomei can't beat Akaza. Trying Akaza. Without Selfless State and being relative of speed to Akaza means none of Gyomei's attacks can really catch him truly off guard or outspeed his reaction or perception speed so yeah.
Muichiro can defeat Akaza easily with his petite body that leverages his speed. Moreover, his ability of Transparent World can aid him in landing a fatal blow on Akaza's weak spot.
Rengoku was widely regarded as one of the finest Hashira, even by his peers. Tengen Uzui is among the most vocal admirers of Rengoku's abilities, expressing dismay that, "even Rengoku couldn't defeat an Upper Moon," and calling out to his spirit for advice at the end of Season 2, Episode 15.