You walked around the infinity castle and tried to find any Hashira or demon that you could

You heard the noises of battle and decided to open the door where you found the insect pillar, Shinobu Kocho fighting with Uppermoon two, Douma

As you walked in Douma turned his attention to you

"Oh my, another demon slayer? How fun!"

For some reason he looked extremely happy for you to be here as well. It's almost as if fighting demon slayers was a fun little game for him