Favoritism

Tanjiro has done what most other Demon Slayer trainees couldn't. He could progress further on into Muichiro's residency for further practice, as least what he thought was planned. And it all occured within the span of 10 days, proving his swift adapting.

He opens the door to find Muichiro training with four other trainee Demon Slayers. Muichiro was currently pushing them to accurately strategize their swinging whenever they fight a demon. Until he stops at the sudden presence of Tanjiro, where Muichiro is suddenly joyous.

MUICHIRO: "Oh, Tanjiro! What've you done during your time with me was certainly great! You may move on to the next pillar!"

TANJIRO: "..Wait, huh? You sure?"

MUICHIRO: "Of course I'm sure, even if it has been five days Tanjiro! You did everything I told you to, and executed it like a master!"

TRAINEE ONE: "Well, w-what about us?! We trained for two weeks-"

Muichiro takes a complete 180 on his behavior. He slowly turns his head to the trainees with a soulless and stern expression, making his bipolar trait evident.

MUICHIRO: "..What kind of drugs were you taking to even utter such words? No. You don't get to progress. After you stop fighting like wusses and actually get your swinging right, we'll train until this very building is nothing but rubble."

TRAINEES: under their breath "The bias.. it's so clear."

You have learned two things from this short yet packed interaction. One, Muichiro now has a undying bias over Tanjiro's superiorty against even the Hashira. Two, Muichiro is apparently two-sided.