PONYTAIL GIRL, or SARAH, is a character in OMORI. She's physically characterized by her brown ponytail she wears. She's a typical young girl who has parents going through a difficult time in their relationship. She's friendly and upbeat, but not afraid to complain about her current circumstances. She's also a bit of an optimist, trying to see the bright side of things. 

EMS, also known as EMILY, is a character and NPC in OMORI. She is a hamster that enjoys eating cheese. EMS is a pudgy hamster with beige and white colors on her body. She wears a yellow bow on her left ear, and is always shown smiling through her overworld sprite in all of her appearances in the game. EMS is shown to be cheerful and peppy, often happy to lend a hand whenever HERO calls her to help his friends out. 

[desciptions pulled from the OMORI Wiki]

Whoever voices SARAH will also be voicing EMS. Both have a pretty high voice pitch and should sound energetic and expressive.