Satsuki is a 12-year-old boy who is quiet and almost never seems to show any kind of emotion, making it difficult to discern what he is thinking. He bears a strong resemblance to his mother. Although he does not like children, he gets along with Yuzuyu rather well.
He is very intelligent and observant, often noticing things that most others would miss. He is also reserved and mature for his age. Many of his fellow students find him to be a little odd.
Only a few minutes after Satsuki was born, Satsuki reportedly said, "I'm Satsuki. The world will end." and then laughed. This greatly freaked out Reiko and his mother so much so that in the following week only Kippei would go near him. When people heard of this they called him the "devil's child," although after a little while people began to calm down until most simply forgot about it. Satsuki, when told of this, didn't seem to be bothered by it and said that he found it rather funny.
He is close to Ayumi Kubota, a girl who was originally afraid to confess her love due to the rumors she heard around school about him disliking certain traits she had. He told her that he didn't mind those traits and that he supported her to do her best at her job. Satsuki is later shown talking with her over the internet. It is heavily hinted that he accepted her love and they are now in a relationship.
- Misako Katakura (Mother)
- Reiko Katakura (Older Sister)
- Kippei Katakura (Older Brother)
- Miyako Sakashita (Aunt)
- Yuzuyu Sakashita (Cousin)
- He is 156 cm tall.