Hashira and Non-Hashira Training Session

It's a clear October evening at a secret training ground hidden in the remote mountains of Japan. The Mist Hashira, Muichiro, needs to sharpen his sense of concentration and mental clarity. To help him, he enlists the assistance of a creative and shy non-hashira, Cupid. During the day, they recall the demon-slaying movements and teachings together. Each of them taking turns in leading the practice, which gradually turns from being serious into fun where they mock fight and laugh, allowing for gentle touches and shared inside jokes.
As night engulfs the training ground, under the gleam of a lantern, they turn to a quieter, deeper practice. Here, they need to trust one another, feeling each other's movements as they execute tight forms and stances, which the limited light makes more intimate and sensual. Throughout the evening they push their boundaries, indulgent in their shared responsibility, finding solace under the blanket of stars.
As dawn approaches, they move into the final, quiet part of their training. They lay together under the open sky, their senses heightened from the day's painstaking training. Here they share their deepest fears and dreams, linking their hearts before they sleep, waking in each other's arms when the first rays of the new day embrace the valley.

First, you need to recreate the Demon Slayer appearance. Searching online for visual reference material from the show can be helpful for dressing up as Muichiro and Cupid, and even possible to create costumes with what you have at home.
Set up the training grounds in a corner of the living room. You could rearrange the furniture, place some cushions around to create a comfy seating/lying spot, and also to act as training obstacles during the day. A lantern or candles can help set the atmosphere during the night, adjusting the light spectrum according to the timeline of the roleplay.
Last but not the least, agree on the safe words and gestures, to respect each other's physical and emotional boundaries during the role play.

To make the scenario more realistic, you can research about sword forms and stances, including the quietest way to unsheathe a sword, or learn about the essential movements for a sword fight.
Incorporate trust-building activities into the training timeline. This could range from trust falls to painting each other’s faces to prepare for battle.
Above all, make sure you both enjoy being in character and stop immediately if either of you becomes uncomfortable.