Quote Originally Posted by Buffalobiian View Post

The whole "mark" thing is also inconsistent now. At first it looked like Tanjiro got it due to his use of First Breathing, but Muichiro has developed something "similar" through gaining his memory, guts, talent and resumably being a descendant of the First Swordsman - but he does not perform First Breathing as far as we know.

I'm also enjoying Kanroji's eyecandy a lot more in the OP/ED than I thought I would.
Aren't they both descendants of the same family line?
Mist Hashira's father has the same eyes as Tanjiro and his crow also mentions that he is a direct descendant of the guy who invented the first breathing style.
Maybe the mark is related to just the bloodline. Or it's related to demon-slayers in general, considering all the breathing techniques are sub-techniques of the first one. Maybe it's a "I'm in the zone" thing.
They look a little different as well. Mist Hashira's is a bit more "flowy" and reminds me of clouds.