Mabel Pines

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Mabel Pines is one of the main characters of Return to the Falls. Hyperactive, enthusiastic, loving and perhaps overly cheerful, She's returned to the world of Gravity Falls feeling distinctly more like an action hero.
Taller than Dipper, no longer wearing braces and finding complete joy in being a teenager, Mabel has arguably changed the least since 2012, but has instead become simply more aware of her surroundings, actions and how they effect people - making her a simply funnier, more pleasant person than ever to be around, despite her childish nature.
=== Return to the Falls ===
After leaving Gravity Falls, Mabel focused on improving certain aspects of herself, and going up to eleven with others. She's become an expert in grappling hooks, obsessed with polaroid photographs, an avid arts-and-crafter and a bit of a flirt, apparently going through many different relationships in the space of only a few months, fuelled by her natural confidence and charisma. 
Fiercely protective, smart and more than a little hyperactive, it's easy to disregard Mabel as a source of quips and silliness - but she's saved the day more than once with her quick thinking and follows a similar strategy of acting first to her Grunkle Stan. For these reasons, while can sometimes be unpredictable, she's indispensable. 
The person who shoves Dipper and Pacifica together the most, Mabel is a voice of reason in the two's relationship when their personalities or individual traits clash, and often acts a mediator (or inhibitor) when things go wrong. She's grown very open about her part in Weirdmageddon and has spoken of her regrets more than once, while nobody with sense particularly blames her for where things went wrong. 
In The Historic Horrors of Family Northwest, she met Kevin properly for the first time, and the two instantly hit it off. They're now practically inseparable and incredibly dedicated. The irony of her being the Pines twin who ends up with a Corduroy is not lost on her.