The main reason why I personally think Sasha can share the same overarching protagonist role with Leif, The Guardian and Valeriana because even though she was an antagonist in two seasons it's just her heroism in Season 3 might make us think that she's not an antagonist anymore, she redeemed herself in the end of Season 2, she's forgiven by everyone, she's the reason why Captain Grime became a better leader to his toad army, her not wanting to support Marcy's interest, manipulating her entire friendship with Anne and Marcy, being the reason why Marcy is the direct cause of her friends to be at Amphibia and peer pressuring Anne to steal the music box is the reason why her, Anne and Marcy went to Amphibia in the first place, she was the reason why Anne was known to be a selfish anti-hero in Season 1, she's the reason why King Andrias found an easier way to invade earth, she's the reason why The Core used Marcy's body, she's known to be Anne's leader in Season 3 and she's the source of all the series' conflict (it was revealed that her not paying attention to Marcy is the reason why Marcy put her friends to Amphibia in the first place).