Monika is 16 years old, the oldest in the club, tied first place with MC

Unlike her game incarnation, Monika isn't actually good at everything, in fact, she isn't really good at much, the best she's good at is swimming, cooking, and maths, though she has a massive amount of optimism and energy (though not as much as Sayori, though to be fare, nobody could be as optimistic and energetic as Sayori), and if she fails at something, she keeps on trying until she does it correctly, and being the leader, she does know how to get the other 3 girls to behave, something that is actually rather tough (especially in Natsuki's case),

Her voice shouldn't sound too old, but shouldn't sound too young either, she's the middle ground