Inside a prostitution house 

A baby with blue hair is currently crawling around inside an empty room. He is accompanied by a little girl around the age of ten. 

"Ah. Don't move around so fast or you will hurt yourself Mitsuki-chan!"(The girl) 

The young girl's name is Chisa. She was assigned to look after the baby by the house madam. 

"Haaaah.... I'm bored.. Why am I the one taking care of you when I can be working with my friends." Said Chisa while pouting 

Meanwhile the baby... 

'Haaahhh???! I don't want you to take care of me either you snotty brat! If I can speak now I would've tease you till you cry long ago! Eii!! Take this!'(Mitsuki) 

Mitsuki proceeds to bite Chisa's arm with his toothless mouth

"Oi! Don't bite my arm Mitsuki-chan! Hahahaha! It tickles! Stop! Puhahaha-"(Chisa)

'Heh, learned your lesson yet kid?'<smug face> (Mitsuki)

<Sweatdrops> 'Wait what am doing right now.... Now's not the time for that!'

'Haah.... It's been 1 year since I reincarnated here. It seems that the black entity have set my birthplace in the entertainment district.'

'Talk about a dangerous place to start. Isn't Daki the uppermoon 6 somewhere here right now?'

Mitsuki proceeds to gaze at a nearby window

'Wait, if I think about it she was missing from the entertainment district and will only appear 1 year after the canon starts..... Talk about convenient.'

'Welp I guess the entity did say about giving me protection for 15 years.'(Mitsuki) 

The girl, Chisa, pouts for being ignored by Mitsuki 

"Kora... Mitsuki, what are you spacing out for. Come here, sit on my lap."(Chisa)

Mitsuki slowly crawled to her lap for some cuddling.

'Damn I can't resist my nature as a baby. I guess even if my mind is mature, my body will still instinctively act my age. It's like I don't have full control of my body.'

'By the way my body feels weird. I can feel that it's not the same as my old body. The air in this world also seems a bit more heavy than my old one.'

'Hmm...I guess that's how breathing technique works. My hair is blue tho..... Is it normal in this world?'

'Well, the black entity likes to mess around with me but I doubt that he'll actually harm me...'(Mitsuki)

'Maybe.... I hope..... I wish....... He won't hurt me right?'(Mitsuki)

'One more thing though, why do my body feels strangely wierd and hot at night? The folks here also said something about me being a special product or some sort'

Little did he know that his body will be one of his source of shame in the future....

'I guess I'll stay here and wait for the opportunity mentioned by the black entity.

 It's not that I can survive outside alone right now. I'm a modern person. I need to familiarize with the culture here and the time gap. 

 

Until then I'm staying here. I am quite unfamiliar with the current timeline after all. Yesterday I saw some people getting amazed by electric lights near the street. I guess 15 years before the canon starts electricity was just being introduced huh.'(Mitsuki)

Meanwhile, Chisa is busy complaining about some disturbing stuff ( by modern standard ofc) 

"Haaah... I hope I can grow up fast so I can start working in the service department and find a rich guy to seduce. Don't you think so too Mitsuki-chan."(Chisa)

'Oi, oi, what is this ten years old kid saying now? <sweatdrops> And one more thing.... What is with the -chan?!

Isn't a boy supposed to be called -kun!

Just because my face looks a bit feminine it doesn't mean you can start calling me with -chan! I hope I'll grow more masculine later......Haah.... am I gonna be ok growing up in this prostitution house???

'I'll need to think of a way on how to get stronger or else I'm as good as dead'(Mitsuki)

"Hey, what are you spacing out for?" said Chisa

'What is she saying?? It's not like I can reply her.'

'If I wanted to survive this place I must become stronger and learn some breathing technique. I'll at least need to be strong enough to fight off the enemy that the black entity send. 

Worse case scenario is that those enemy will disturb the original story in this anime world. Wouldn't want the demon slayer corps to die right?

 I mean who else is gonna solve the demon problem if not them? 

What? Solve it yourself you said? And risk dying everyday? Huh? No thank you. I already got my hand full with the black entity' 

'Im gonna try to live my life away from the main cast! Banzai to my mob life!!' (Mitsuki)

................................................................................

Somewhere in the afterlife 

"I can't have you doing that can I?" Said the black entity while grinning

"I'll need you to be involved in the main story or else it would be boring for me"

"Well... time to spice up things a little bit shall we......"

The Black Entity grinned ominously