Well! Time to get to it I guess. Here, we start with the easiest to understand and the almost most annoying thing to me.

OOC - Out Of Character.

"Karma, well-known throughout the school for his violent behavior and troublesome attitude. He is quite cunning, mischievous, and somewhat sadistic, usually playing foul, tricking, and at times manipulating, humiliating and fighting others.

Despite his somewhat sadistic nature, Karma actually has good values behind his motives, and can be quite empathetic. The reason he was placed in Class E was that he had rescued a senior from Class E being bullied by students from the main building. He is, unlike his former classmates, unprejudiced against Class E. He believes each and every one of them have their own special talents and will defend them, especially against Class A." -AC WikiFandom

He's not always going to be extremely nice, clingy, or lovey dovey— but that doesn't mean he's a straight up murderer either.

I've seen fanfics where people have gotten Karma's character/personality spot on, but there's just a lot that rubbed me the wrong way. There have been fanfics where Karma falls almost instantly with the reader when she walks into class, or he's overly mean to her which isn't like Karma at all.

I would think Karma would be the type to not really care about the reader until they did something that caught his interest. Until then, reader would just be another classmate to him. When Karma DOES take an interest in the reader, I think he'd start off by talking to them or teasing them just a little bit more than he usually would. He isn't the type to open up easily, so he wouldn't go telling the reader everything about himself like how lonely or neglected he is by his parents. I've noticed that he's a subtle person when it comes to feelings most of the time and rarely is he blunt about his emotions.

Another thing... Karma also rarely cries, which I've only seen him cry once in the entire anime which was when Korosensei died. So when the reader is upset or hurt, I don't think Karma would be the type to cry seeing them that way. (If they were to die and he cared for them deeply then it would make more sense.) He isn't exactly the best at comforting either, since he never really had anyone console him when he was going through hard stuff, so seeing people write him as a "soft boy" is kind of weird to me.

Now, there was a book I read that had a reason for a sort of soft-ish personality on Karma. He was in a relationship with the reader before they broke up with him because their parents forced them to. And as chapters went by, he wasn't in every single scene with the reader, nor was he overly clingy except for the parts where it hinted at Karma still having feelings for the reader. The reader and Karma still held these feelings for each other, so it made sense why they'd treat each other the way they did that wasn't in a way normal classmates would act. It was executed quite well, and I honestly enjoyed the book.

There could be another reason for Karma being a little nicer than he would be to normal classmates and that'd be if the reader was a childhood friend of some sorts. But even then, I actually see Karma teasing them more since he's known them longer.

NOW, just because Karma isn't super nice all the time or super romantic, DOES NOT mean that he's a bully or a murderer. I'd describe Karma as a Good-Bad Boy. He's not a bad bad boy, but he's not a goody-two-shoes boy either. He's Chaotically Neutral.

A few examples of book plots I've seen that absolutely do not fit Karma's personality whatsoever:

- Karma physically and mentally bullies the reader into near suicide many MANY times.

- Karma kills someone saving the reader. (He's a literal minor and even if he's being trained as an assassin, I doubt he'd ACTUALLY kill someone. I don't think some people understand how detrimental murdering someone is.)

- Karma is a psychopath or sociopath but somehow falls in love with reader. (Doesn't make sense since I read that psychopaths and sociopaths (for the most part) usually use people for their own gain and not be with them as someone they love.)

- Karma is overly nice and flirty or even perverted towards the reader. (Karma doesn't have a flirty personality at all in my opinion and he isn't perverted like Maehara or Okajima, so I don't get this trope that's commonly used in Karma fanfics.)

All in all, if you want to write Karma's character, I would watch the anime or read the manga and study what Karma says and how he says things in certain situations. And PLEASE understand that people being straight up murderers aren't really supposed to be romantic...

Update: I forgot to add this, but I've seen a lot of people commenting asking if Yandere AU was acceptable and yes. If you wrote a Yandere AU Karma then it makes total sense, but try to keep large personality traits true to his character or it'll feel like an OC Yandere instead.