Yoi was born mute, so he was incapable of speaking from birth. This diability would result in him having to often write down what he wants to say, otherwise the most he can do is make (admittingly adorable) faces at people to communicate his thoughts. He didn't always have that many friends growing up, as even with his exceptional writing, it was hard to make friends that way without speaking abilities. Yoi wasn't always bothered by this, as he would still try to stay positive and friendly with all those he met, and enjoyed listening to people when he could get the chance to.

It wouldn't be until his family moved to America where Yoi would start to make progress on befriending people. He would meet the Nekogamis, forming a strong bond with Aku the most, and eventually he would settle into a relationship with Yumi after Sophie introduced the both of them to eachother. Yoi enjoys listening to Yumi's smart talk, and she enjoys reading the transcripts of stories he comes up with. It's quite a peaceful relationship between the two of them enjoying their company together.