Tsukumo was adopted when he was very young kid - in 1999 - by the Sakurazuka family following the loss of his family because of a natural disaster in 1996.
At first, he couldn't think of
Natsuki as his sister until she saved him - the same year - from drowning, getting hurt herself in the process. Ever since, he has been devoting a deep love to her, more than brotherly.
Tsukumo rarely loses consciousness after a fight, even if it was a difficult fight. On the other hand,
Mimi's kitchen nearly sent him to the other world ...
(spoiler hidden)