This is going to be a long and complicated one.

You basically can't be involved in fanfiction and have never encountered a headcanon of some kind before.

A headcanon is kind of a hard thing to explain. It's kind of like taking an idea for a fandom and treating it as fact.

If you're still confused, let me give you an example of an acceptable headcanon.

Say you headcanon Lillie as a lesbian for example. You then treat Lillie as a lesbian whenever you talk or write about her.

Now let's say someone says "Why don't you ship Aureliashipping?". Instead of arguing for another ship or disproving Aureliashipping, you would say "I headcanon Lillie as a lesbian, so her and Ash would never work for me", and that's why you don't ship it. There's nothing wrong with that.

That is until people start arguing (or debating). You can't really use a headcanon as a talking point in place of real evidence, unless of course there is evidence to support it. I can scream "I headcanon Ash and Red as brothers" or something else all day, but unless I give evidence, my headcanon can't be used to disprove anyone on their talking points.

There is also the much darker side to headcanons. I recently saw a Gladion x Lillie fanfic while scrolling through Wattpad. Pretty disgusting stuff.

Not to worry, says the author in the story description: he headcanons them so that they aren't related.

Now here's why I say that isn't enough: the reason you would ship two people together is because they have characteristics that are the best compatible in your opinion. When you ship people, you don't just ship the physical aspects of them, you ship the abstract of them too. The idea of them.

Headcanoning Gladion and Lillie to not be related to ship them is essentially saying: "Yeah they're related, but I still ship them I'm just going to say they aren't so people don't look at me funny."

You cannot deny that you still ship them even though they are canonically siblings. It's just an obstacle to people like that.

With that said, who knows how far people take headcanoning to satisfy their sick desires. I don't even want to think about the almost guaranteed pedophilia fanfics out there that headcanon as an excuse.

All I'm saying is that fandoms need to be careful of what they do and post. Write responsibly, kids.