Satoshi Morinozuka is Takashi Morinozuka (Mori)'s younger brother. He is captain of the middle school kendo club and is also a member of the karate club, of which Yasuchika (Honey's younger brother) is captain.
Satoshi is first introduced with Yasuchika in chapter 29, after Honey mentioned to Haruhi during the cultural festival that both he and Mori had younger brothers.
Compared to his brother Mori, Satoshi is almost identical in appearance (although notably not as tall as Mori), but is considerably more talkative and outgoing. He is also a devoted member of the Morinozuka clan, going so far as to hit Yasuchika with a stick when he is rude.
Satoshi has shown that he does care for Yasuchika however, as after he caused Yashuchika to cry, Satoshi was quick to apologize. In contrast to Yasuchika's disdain for his older brother, Satoshi adores Mori even though they are so different.
In a small part of the manga, it shows that he has a liking for spicy things after the twins put hot sauce in his tea.
Satoshi has only appeared in the manga a few times, and has never appeared in the anime.