Mrs. Satou is Lilly & Akira's Scottish Mother.  While it's never mentioned in the game itself, the devs have stated that her first name is Karla. She doesn't have much of a part in the game beyond a phone conversation. Prior to her time in Scotland she was a journalist, and at some point she met a Japanese businessman who is revealed to be Hiroyuki Satou. The two eventually got married and moved to Japan. Here their first daughter Akira was born. After seven years their second daughter Lilly  was born. Twelve years ago they lived together like a normal happy family until she and her husband moved back to Scotland when her husband's business required them to relocate. Lilly and Akira chose to remain in Japan.

So now that you're caught up on the lore, you should know that her voice should have a Scottish accent, and should sound polite, reserved, motherly and kind. Her voice is somewhat like Lilly's except with the accent, and the fact Mrs Satou is much older.