
Whats the definition of a doujinshi? I used to think it’s just a not canon manga xD but am i wrong?

From what I understand, doujinshi are self-published works. this means that doujinshis can be written by anybody (the original author, fans, others artists, etc) that are about the characters & story of the work. djs are usually canon when written by the original author--but there are time when original author doesn't make it canon but just play around with their characters bc they think its interesting. on the other hand, anything made by other creators relating to the work is not canon. so the answer to your last question is YES. It's possible for original author to publish their work w/o a company and tat is what a dj is.
For this doujinshi, it was made from the same mangaka of the main story and it is canon. instead of publishing with the original story, this dj was published separately.

I have a question concerning the prequel:
When Rogi forced Tatsuyuki to have sex with him. Technically, It wasn’t rape, it was more like Tatsuyuki selling his body(because he owed money to Rogi and didn’t want to be a burden for his dad). But then right after Nozomi took Tatsuyuki out of Rogi’s house, they were pretty much acting as if Tatsuyuki was kidnapped. Did i miss something?(i’m so confused)

Blackmailing someone to have sex with you is generally not a nice thing to do! Legally, it's called "Coercion", and in some states it's a rape case (it should be one everywhere, but such is the world we live in). Coercion by itself is illegal too! IDK about the laws in Japan though, not that anyone in this manga's gonna use the legal system for anything!

Are you aware of what rape is? He *didn't* give him a choice. It wasn't consent, it wasn't a choice. He was blackmailed and therefore in that regard forced to sell his body. That's not consent and it's not in the least legal. That is rape.
If you're telling someone you will kill their family or in other ways hurt their loved ones if they don't pay with their body, is that not rape, if the person agrees? Someone threatening and you agreeing to avoid said threat from actually happening is not consent.
So Rogi is in fact still a rapist scumbag - and he knows.

Ok so he made the uke feel guilty on purpose but was he depressed for real or not?

he never was. if you're referring to his "crying" scene where he acts like hes sorry or remorseful in any way, that was an act. all of it was an act. he wanted to make him feel bad so that he would come back in the end. he's real cunning, isnt he? harada's works almost never have happy endings, though i think this story is one of my favorites

it isnt said explicitily so i dont think anyone can have the right answer except if the author says it. I know that he was planning to use the guilt as a mean to get the uke back but maybe he really was sad(since he thought he had lost the uke) and so him crying could be real. And it is possible he was really depressed(he had been pursuing the uke for very long so thats understandable) but he made sure that the uke would be aware of it. I am really not sure though, both opinions seems very possible which is why im asking others. And harada's works arent all that bad. I saw atleast 3 of them with good endings.
And yet this « coward » is the same dude that expressed his feelings quite easily towards his last crush. It’s a bit disappointing, the story is just taking longer without anything interesting and I don’t understand where the MC put the confidence he had at the begining :/
Public self-confidence doesn't mean actual high self-esteem.
Can i high five you?