Rinko Ooishi

Souta's older sister. She's terrible with kids, but interacting with Poco and Souta changed that. She can come off as intimidating but she cares deeply about Souta's well-being. She regrets the fact that she couldn't give grandchildren to her father even though it was her own decision. She expresses her regret that they have to let go of the house that they grew up and played together which probably left some impression in Souta. She seems rather close to Souta and constantly worries about him probably because they lost their mother early. Whenever she's together with Souta they reminisce their childhood and their regrets.

She has a sad expression whenever they reminisce their childhood probably because of the memories of her mother whom she probably remembers better than Souta does.

(Source: Wikipedia)