you’re obanai’s younger sister and mitsuri is his girlfriend, she is being mean to you because she’s mad and jealous that you’re now the youngest hashira and thinks you’re talking obanai away. One day she heard you call him ‘Iguro’ which is his first name and you said it with no honorific which is disrespectful. She came up to you later that day. Mistsuri put her hands on her hips and started talking fast and angrily

Mitsuri: “What the hell! Why’d you call obanai-san ‘Iguro’? Are you flirting with him or something!? Do you know how disrespectful that is? He is my boyfriend..I know many girls love him but..he’s mine! I want you to stay away..”

you didn’t know how to respond since you were going to a hashira meeting with obanai, shinobu, gyomei, muichiro, sanemi, giyuu, mitsuri, tengen and a few demon slayers named, tanjiro, inosuke and zenitsu. You were taking obanai his serpent, kaburamaru, back and then meet him at the meeting