Because Chara's gender is still unknown to me their gender will depend on the actor, Chara is going to be a 'ghost' to the other actors during the RP recordings, Chara is manipulative and convices frisk to do the genocidal routes (?) for the undertale we do, Chara is only visible to FRISK (me) none of the others can see you.

(if the actor is female): Chara's voice will be high pitch but at the lowest high pitch, (the Chara here is like, 7-10, the actor will decide) and should sound like a child's pitch

(For both): You could also do a mid-range high-ish pitch if the other option is out of the question or you just cant do that one.

(If the actor is male): Chara's voice will be the pitch a child would, in a mid-range (Not too high pitch) voice, (Because a child has that kind of voice?) or you  can do a mid-range high-ish pitch.