Kaidou is very competitive with Kukai, which shows when Kaidou asks to play soccer with him. He is shown to be very harsh, but deep down in his heart hides a good person. He is similar to Kukai.
Kaidou is the most muscular of the five brothers. He has dark brown spiky hair and also sideburns. His eyes are brown and he has light tan skin.
- Grandfather Souma: The brothers have a grandfather living in a temple on the mountains.
- Shuusui Souma: Kaidou's younger brother, and also the second eldest brother of the family.
- Unkai Souma: Kaidou's younger brother, and also the third eldest brother of the family.
- Rento Souma: Kaidou's younger brother, and also the second youngest of the family.
- Kukai Souma: Kaidou's younger brother, and also the youngest of the family.
- In episode 79, Kukai said that he had to buy groceries because he lost in a game of rock-paper-scissors. It is most likely that the game was between Kaidou and Kukai, and that Kaidou was the one that challenged Kukai in the first place.
- "Kaidou" means "child sea" in Japanese, and has only one kanji in his name the same as Kukai's name. Meaning: Kaidou and Kukai.