An idea that into my mind since season 5

Heather always had eyes for Astrid. Always.

In the beginning Astrid was the only to see Heather through her act and Heather had secretly loved toying with Astrid's emotions. Riling Astrid up had turned Heather on.

When they met again, Heather was very anxious to see Astrid again. It was a pleasant surprise that Astrid and her got along so well. They laughed openly together now and spent some quality time bonding.

Heather noticed that when Astrid spoke of Hiccup, the girl would blush. It broke Heather's heart but she only wanted Astrid to be happy in the end. So she ended up teasing the younger girl about Hiccup, playing along with the whole 'boy talk' even though in reality Heather only wanted Astrid for herself.

When they said goodbye, Heather knew it was for the best. She needed to figure herself out and put some distance between herself and Astrid.

It was like they were fated to meet again on Ryker's ship.

She and Astrid played everyone for a fool with their act and Heather even managed to convince her to keep their friendship a secret. Especially from Hiccup. Not only would he interfere with her plan because of his hero complex but Heather wanted to keep this secret alliance with Astrid between them. She didn't want any third party, especially not someone who was competing with her for Astrid's heart.

Heather could tell that Astrid really struggled with lying to Hiccup. Heather felt guilty for doing that to her, putting her in this impossible situation, but it was important that her plan followed through seamlessly.

They had their secret meet-ups and every time, Astrid would suggest they tell Hiccup. It annoyed Heather slightly that they couldn't spend time together just the two of them without Hiccup being constantly brought up. But she endured it for Astrid's sake. She knew the flaxen haired girl was in love with the Night Fury rider. It wasn't that Heather disliked Hiccup, she thought of the young man as a brother of sorts. But there would always be this bitterness that he stood in the way of Astrid and herself.

Hiccup was eventually let in on the plan and Heather pushed aside her feelings for the greater good. There were a lot more at stake and she needed to focus.

After Dagur freed her, Heather decided to leave straight away. Without as much as a goodbye.

She was conflicted. Over her family history, Dagur being her brother, her feelings for Astrid and for betraying Hiccup who had always been a good person, so kind and treated her on the same line as the other riders even though she wasn't even a part of their group. He was a decent person and she was trying to ruin his relationship.

During her time away Heather started to get letters from Fishlegs. Heather did like Fishlegs a lot. He was kind and sweet. But just like with Hiccup, she only thought of Fishlegs as a platonic friend. There could never be anything more between them than friendship them.

Heather was invited to the Edge and reuniting with Astrid again was the best part. Just like her last visit, Heather couldn't help but notice how close Astrid and Hiccup had grown. She decided to tease Astrid about it and urge her to confess her feelings. Astrid in return teased her back.

Heather still didn't feel all too comfortable. Everyone tried to make her feel welcome and despite the early obstacles she ended up staying. Fishlegs wasn't Astrid but he had her back and Heather could really use a friend.

During all the missions, she and Astrid would team up and Heather even moved into Astrid's hut. Now that they were living together, Heather fell even more in love. At night she would stare at Astrid sleeping and wonder if the girl could ever love her back?

She was in the friend-zone, watching Astrid making moon eyes at Hiccup. It was painful but Heather kept up her mask.

When reunited with Dagur again at last, she knew from the get-go that it was to leave. Hiccup and Astrid were falling even more in love and Heather couldn't bear to watch her. She had enjoyed the Dragon Riders, being roommates with Astrid and having a good friend in Fishlegs who she had grown every fond of.

After moving on, Heather dropped by to visit Astrid one day when she suddenly saw her and Hiccup. Kissing. Wrapped up in each other.

Heather sighed to herself and left immediately. She always knew Astrid loved Hiccup but it still hurt. It was time to let go.

After Hiccup and Astrid became official, Heather distanced herself even further. She still thought of Astrid as her best friend but they were not as close as they used to be. Heather couldn't afford the heartbreak.

Now at last Heather was leaving the Archipelago with her brother to move to a faraway land. It might be the last time she saw Astrid. They had gone from enemies to allies, to best friends, to roommates, to a deep and indescribable bond. Astrid was the most significant part of Heather's life and now she had to leave it behind. She had to let go of Astrid for real this time.

They met one last time at the cliffside. Astrid was now bethroded to Hiccup, officially set to marry him.

Heather held back the sob and forced a smile.

"Don't get forget to invite me to the wedding," she her unrequited love one last time.

Astrid laughed and hugged her goodbye.

The next time she would meet Astrid again, she would already be married.

Heather flew off with Dagur into a sunset but this time she didn't sneak a second glance at Astrid. It was a good thing she didn't as the sight of Astrid in Hiccup's arms would have broken her heart anew.

"Goodbye Astrid," Heather whispered and Dagur shot her sympathetic glance.

"What?"

"You loved her, didn't you?"

Heather laughed. "It was that obvious?"

~The End