JOY (also known as SMALL GIRL) is a character in OMORI who can be tutored. She appears to be somewhat pale and short in stature. As JOY, she is extremely shy and nervous, not confident. Her father keeps her pretty focused on her studies and limits her time outside with friends. Her counterpart, HAPPY, is more boisterous and chipper, no trace of stutter or shyness in sight. [description pulled from OMORI wiki]

As JOY is pretty young, a medium-high to high pitch voice would be good for her. When you are voicing HAPPY, the only big difference besides sounding a little younger, is that there is no shyness or hesitancy.