Zenitsu and you have know each other since childhood (don't ask how -_-) and you both have grown on each other and you always get jealous of all the other women that he tries to flirt with obviously although he only does that to make you jelly. After a few years of you knowing each other you met tanjiro and inosuke now you all go on missions and hangout with each other but zenitsu does get a little jelly sometimes.