Astrid is also a possibility. She's a strong - willed character, but perhaps there's more to her than what we usually see. She could be hiding her softer emotions or insecurities behind a tough - girl mask. For example, she might be secretly worried about her relationship with Hiccup or her place in the tribe.
Another option could be a new character introduced specifically for this fanfic. This character could bring a fresh perspective and their own secrets hidden 'behind the mask'. They could interact with the existing characters in interesting ways, causing the other characters to question their own masks and what they're hiding.
It could be Sasuke. He might be wearing a mask for some mysterious reason, like hiding his new powers or a new allegiance. Maybe he's on a secret mission that even Naruto doesn't know about at first.
It might also be a completely new character created for the fanfiction. This new character could be wearing a mask for a unique reason, like hiding a special power or being part of a secret organization. The fanfiction could then revolve around this new character's journey and how the mask plays a role in it.
It could also be about exploring the characters in a deeper way. Maybe the fanfic delves into the insecurities or past traumas of the characters that they keep 'behind the mask'. For example, a character who seems strong on the outside but is actually struggling inside. This adds a layer of complexity to the story.
Sasuke could be a great candidate. He has always been a bit of an enigma, with his brooding nature. He could be hiding a new power or a different side of his personality behind a mask, either self - imposed or as a result of some outside influence. Maybe he's trying to hide his true intentions from Naruto and the others as he pursues his own goals.
It might also be a new character created specifically for the fanfiction. This character could be someone who has infiltrated the Hidden Leaf Village, living behind a mask of false loyalty while having their own secret motives and goals.
It could be a rogue vampire. You know, in the Twilight world, there are vampires who don't follow the Cullen's rules. This 'man behind the mask' might be one of them, lurking in the shadows and causing trouble for the main characters.
A new character could be the main character in the HTTYD runaway fanfiction. For example, a young girl who has just become a dragon rider but is afraid of her new responsibilities. She runs away to avoid facing them, but during her time away, she bonds deeply with her dragon and gains the confidence to return and take on her duties.
Sasuke might be the main character. Since he's always on the hunt for power, he might come across this strange mask. And when the mask starts to 'eat' his face, it would be a test of his willpower and his ability to handle the unknown and dangerous forces that come with it.
Astrid could also be a main character. Maybe she runs away for some reason and her dragon Stormfly is with her. Or she could be chasing after Hiccup and Toothless if they are the ones on the run. There are also other Vikings or dragons that could be main characters depending on the specific plot of the fanfic.
It could be Naruto himself. Maybe he discovers the mask and when he puts it on, it causes a big reaction among his friends and foes.