The truth of the matter is that Hiccup has never been into Heather and nor has she been into him. She just took advantage of him being a nice person and manipulated the situation in her favour.
Hiccup is a nice guy by nature, and he is nice to everyone, even his enemies sometimes. Heather was not any special case to him. If Heather had been a male, Hiccup would be just as nice. There was never any crush or feelings. How could there ever have been? Heather changes her personality constantly and everything with her is an illusion. A big fat life, because Heather is obsessed with lying to people.
After she grows up, Heather is still sly, deceive, two-faced and phony. Just more unhinged now.
After she parts ways with her parents, the first promise Heather makes is to ask of help instead of resorting to lying and attacking the riders.
When the riders meet her again three years later, the first thing she does is attack them, even putting their lives in severe danger. When confronted about it she tells them she was only endangering and hurting them for their own safety. It's her way with dealing with confrontation. Making up excuses all the time. Heather can't be anything but the victim at all times.
Out of everyone Hiccup is really put off by Heather's change or what he believes to be a change. The fake sweet girl she portrayed herself to be is no more. Instead he is seeing the real Heather for the first time. The one who is selfish, manipulative and unhinged.
He doesn't believe or buy Heather's lies, and instead decides to interrogate her.
When he confronts Heather about her attacks on innocent ships, she threatens with leaving the Edge. Heather loves to guilt trip people, it's another specialty of hers.
Then comes the next part as Heather yet again manipulates the situation in her favour.
Hiccup tries to tell her she doesn't have to do this alone since he is a nice guy and want to do see the good in everyone. Instead of telling him the truth, she hugs him. It's to make Hiccup shut up and stop interrogating her.
The hug is stiff and awkward for Hiccup. Hiccup doesn't buy it to be genuine and is leaning his head away as well as his keeping his hands as high up as possible. It's the type of hug two strangers would exchange, not two good friends.
Heather pretends to be oblivious about Hiccup's stiffness. While Hiccup ends up grimacing and sighing in disappointment as he realizes it's all just another distraction from the truth. Heather is still willing to put on fake acts even after all these years.
It is to be noted that Heather calls Hiccup a good friend but the fact is.. they were never really friends, let alone good friends.
Heather didn't know him long enough to become a friend, nor did she know him very well. Heather stayed on Berk for less than three nights and the whole time she was putting up innocent fake acts and being two-faced.
Heather and Hiccup are not even good friends. There is no emotional bond between them. No serious connection.
It all just formalities. An act on the surface.
So why did she call him a good friend then, other than to change the subject and thank him for his hospitality? Well, to set some clear boundaries. She is acting grateful to him but also doesn't want him to mistake it for anything else.
So Heather basically friendzones him.
Which is unnecessary on her part since Hiccup wasn't even interested.
He felt sorry for her and later suspects her to be his sister for nearly the whole day, which is the amount of time it takes him to travel to Berk and confront his father about it.
So any inkling chance of a romantic connection is all but washed down in the drain. Even after Hiccup finds out that Heather is Dagur sister, the fact that she almost become his sister, can never change. It's to be forever ingrained in his mind.
From this point on Heather is the girl that almost became his sister.
Heather as well starts to see Hiccup as a little brother. In Dagur's letter, Hiccup is called Heather and Dagur's little brother. Our brother, it says. Not my brother but our brother.
This means that Dagur realizes that Heather has started to view Hiccup as a brother.
All the lying, manipulation and games aside, Heather ends up becoming the closest thing Hiccup has to a sister.
- A oneshot study written by my older sissy of why Heathercup is our NOTP.
She explains; there is a good reason it is not canon and why so few fans like it. It's too forced and fake, especially from an emotional standpoint. Two employees from the Race to the Edge crew even said, Hiccup and Heather have no deep connection and Hiccup only acts nice to be hospitable. He also said Hiccup has never ever been romantically interested in Heather and after re-watching the entire series, I agree with him.