From the When They Cry Wiki:

Rika often seems wise beyond her years. Childish Rika has a tendency to speak using "cute" phrases; including sound effects and nonsense words such as nipah, mii, cat sounds and saying pachi pachi (clap clap) while clapping her hands.  When revealing her True Personality, Rika's cuteness disappears, and she turns cynical and more mature. Since this is the persona and voice Rika uses when thinking to herself, narrating events, and when speaking to Hanyu, it can be assumed it is her real personality, which she keeps under wraps to avoid scaring her friends.