I'm thinking Kaguya from Kaguya-sama: Love is War. Unlike most introverted protagonists, she actually tries, even if her fear and pride still cause things to advance at a slower pace than the characters might have preferred. But she tries, and her ridiculous plots are fun to watch unfold.

I like Sawako from Kimi Ni Todoke

Does Hachiman from Oregairu count? He might for into more misanthropy than introversion, but I like that he's capable and the non-standard way he goes about finding solutions.

Oreki from Hyouka seems like more of a standard introvert that gets dragged out of his shell by an extrovert. He's depicted as intelligent and capable of lateral thinking. I adore how his realization about his feelings is done.

Komi