Tanjiro = Monjiro
Zenitsu = Monichi
Nezuko = Nutella
Tanjiro's younger siblings (Takeo, Hanako, Shigeru, and Rokuta) = Mini Monjiro's
Aoi = Owie
Kanoe = Canoe
(Miss Kocho) Kanae = Canyay
Miss Tamayo = Miss Tomato
Kyojuro Rengoku = Aniki
Senjuro Rengoku = Aniki Jr.
Tengen = Lord Tengen
Giyu = Gayu
Shinobu = Butterfly
Obanai = Openeye
Mitsuri =Bubblegum
Muichiro = Munchiro
Sanemi = Salami
Gyomei = Gyoman
Genya = Getya
The list will be updated when/if new characters and nicknames are added to the story !