
Day one.
Derik thought that his body was different.
"Maybe I'm dreaming…"
When night came, Derik screamed in pain. Memories kept pouring inside his mind. He perceived that there was another consciousness inside him.
Day two.
Derik faced the mirror.
"Perhaps I'm insane?"
He didn't know why he had blonde hair and blue eyes and the memories he received felt real.
Day three.
Derik realized that his consciousness was merging inside the original's body. The feelings and emotions of the original were assimilating inside him.
Day four.
He learned that the name of this body was Christopher Mellwood.
"Christopher Mellwood…Why does that name ring a bell? Mellwood, Mellwood… oh no no no. Please not."
Day five.
Derik realized one thing.
"I'm inside of my novel, right?"
(Hello, I'm your amateur writer, Pujimaki. Hope you like this series! English is my second language so please forgive me..., cause I will make a lot of grammar errors. )
Author! Be careful! Maybe your inside of a your novel!
i read manny novels like that but i have hope for this maybe gonna be a surprise
Interesting
His reaction on Day Four made me really curious to continue reading!
This seems like a prologue, but more like a quick sequence of snapshots, glossing over setting/character, and just focusing on the plot 'housekeeping'. I have the impression this will continue to have a single POV, 3rd limited - which is something I enjoy!
(Non-native English? If things are consistent (names, etc) I'm not really worried. Nicely done!)
Hmmm... He didn't check his face on his first day? He should already do all of those stuff - Stuff that he had done for five days that was written above - when he woke up on day one. Maybe you can put some more explanations why he only noticed some changes day by day. Is he disabled that he can't get up on his bed? Is he dumb that he only noticed on day five that he was inside his novel?
Well.... Chapter zero was.... *cough* just nonsense that I created within ten minutes so.... Please forgive this lowly author!!

He perceived that there's another consciousness inside him.
There was
The feelings and emotions of the original was assimilating inside him.
Here I don't understand what you want to convey. Is he being assimilated by the original or the opposite?
That was is throwing me out of my mind. FeelingS and emotionS are plurals, which neans were. But as I said the whole sentence is not clear.
He assimilated the original.