For this, you will be voicing her younger self as well as her present, teen self!

On the surface, Miki presents herself as a kind and responsible person. Being the representative of her senior class, she likes to take the role of a leader, even during casual outings with friends. Despite her polite demeanor, Miki can act rather conceited, often judging others in a self-righteous manner. This can be seen in the way she treats her peers, tending to act nicely towards them while holding negative feelings that she won’t express until moments where she feels upset or threatened. Her self-image and how she is perceived by her peers is her greatest insecurity, causing her to put emphasis on how she looks and the efforts she makes as a student. 

Because of this, Miki lacks self-awareness, not understanding the flaws in her behavior and the harm it can cause. She often denies or ignores her own wrongdoings, though it's not a malicious act. Even when reflecting on the past, she continues to view herself as a victim. Despite her negative traits, she has the capacity for genuine kindness and sincerity, which she tries to embrace towards the end of the series. In particular, she treats her friends more supportively and compassionately than previously.

If you could sound similar to her English VA, Amber Lee Connors, that would be great, but it is not required!

English VA Example: https://youtu.be/u8Ip6KBDdQY -Spread throughout this video is her younger self. I couldn't really find many clips of her older self talking.