Karin did overreact by telling her friends. Pushing Riona back as reflex was logical if she wasn't feeling like going beyond that, BUT it was a private moment and Riona obviously wasn't forcing herself on Karin since she immediately stopped after Karin made it clear she didn't want that.
"Unwanted advances", that is a dangerous way to simplify the situation. They were in a private, intimate moment and kissing, and while doing some intense kissing Riona just caressed Karin's chest. Riona can't read Karin's mind to know that she wasn't feeling like going beyond that point. By the same logic Karin might have pushed Riona back when Riona tried to kiss Karin and then go tell her friend but Karin just decided that the hand on chest was the dividing line, which Riona had no way of knowing unless specifically told so.
Going outside of manga world, that's why some women complain that men don't try to "woo" them anymore and that men give up "too easily" nowadays, and that's precisely because you don't know if you are gonna be called a rapist for even trying to go for a kiss without first asking the other party to firm a consent form.
They might have been playing pretend lovers but the same rules still apply. Talk things through with each other instead of ruining the other's life for something that was obviously not done in bad faith.
So yeah, fuck Karin.
You haven't explain in a single bit why Karin telling her friends was a overreaction. As established in the manga the Karin was starting to learn about her bodies and to feel weird about it, one of which including feeling gross about it especially in regards to sexually body parts. So her seeking help or advice from friends is completely justified because they are people close to her. It would be a complete overreaction if she told the police, teacher or parents but she didn't she told her friends. You can make the argument that she should have expected gossip to spread around but that doesn't mean action of it was wrong just her not watching out for gossip and rumours. Private moments don't mean they can't be told. The logic of because it's was a private moment meaning she can't share doesn't work if someone is in a emotional situation where they're one of the main parties involved. She talked to her friends people she probable known for a bit and wanted to tell them about it so she at least get some ideas or help from them. That common thing of seeking comfort from friends. I'm putting intent where it meant not necessary be but it still doesn't change that she wasn't telling her friends so they can tell other people to hurt Riona.
Unwanted advances is in no way a dangerous way simplify because that is exactly what was happening she didn't want to have her breast touched. Unlike you who did simplify a situation into by saying Riona only caressed Karin's chest when Riona reach her hand directly into her shirt without warning because heaven forbid Riona ask if she can touch Karin. And this can be explain by Riona being young and not realizing or picking up on things which is way she stop because she realizing she did something wrong or bad. But here a question is then Karin a mind reader? Because how is Karin suppose to know that Riona wasn't forcing herself onto her or going to stop if she said no. You can't make that argument of Riona not knowing that Karin wasn't going to like it unless she is a mind reader if it can be flipped and told from Karin perspective. Also to the point of Karin's dividing line is so weird. If it was a kiss she pushed back then we have a different situation that you can't put a one to one to. Because there a big difference in circumstance between no intimacy to intimacy and non-sexual romantic action to sexual romantic action because kissing using tongue isn't sexual all the time. If you want to make the argument how should Riona knew that is odd because for most people that is very obvious. Kissing is fine anything more that needs to announced or given permission either with words, actions or body language. Riona had a excuse as previously stated but that still doesn't make her actions of I'll repeat reaching her hand into Karin shirt okay just a understand mistake.
What are you on about men feeling sacred to make moves or advance onto girls because they'll come off as rapists. Because how you're phasing it makes it seems like that's exactly what they are by trying to go for kiss won't consent. This is same shit about consent culture ruining dating culture. In no way asking someone if they want to kiss is weird. This can be extended to every intimate act without exception. If a girl or anyone is weird out by that, it's a problem with that person and you're better off moving on. Asking for consent isn't hard and you can even do it non-verbally or through euphemisms if "hey, can I kiss you" is so hard. If someone is struggle with that then they can just look it up.
To that finally point Riona should have asked and you shouldn't blame everything on Karin because her action were understandable because how was she supposed to know it wasn't done in bad faith. And Karin was in the wrong, as I said in the my first message, because they should have said something quicker to stop the gossip or told her friend not to gossip about it.
