hi
RENGOKU USES SUN BREATHING!!![]
well this is just a assumption but....
rengoku is the Flame hashira and has a red Nichirin blade with "destroyer of demons" etched on it.
you see that rengoku has a black sword with a red design on it similar to Yoriichi's, but there is a flame design instead! THAT IS A CLUE! you see Yoriichi's sword is black but red upon contact, the same might be here, he might use the bright red nichirin blade ability. I don't know if this is an easter egg but when tanjiro stops his wound by using total concentration breathing, rengoku san appreciates it and says it will unlock many things.....you know where this is going.. he might be using a refined version of hinokami or somewhere between hinokami and sun breathing. also if there is a guy who read 'ALL' the manga including the event episodes, if i'm Wrong pls send pics of that img where i'm wrong and prove it. If i'm wrong i guess this theory is wrong... byeeeeeeeeeeeeeeeeee.
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my fav pic