|Anime debut||Valiant Fight|
|Japanese voice actor||Daisuke Ono|
|English voice actor||Rob Mungle|
Sasaki (佐々木) is the leader of the rival gang that opposes Kazuto Miyazawa's gang. He is a guy that looks imposing and grave but is actually fair and shows respect for others.
In Valiant Fight, when Yukine and Tomoya visited the hangout of the rival gang, Sasaki proposes a one-on-one fight between the leaders of the two groups near the river bank in order to avoid a full-blown gang war. Since Sasaki's group thinks that Youhei Sunohara is Kazuto, Yukine's group sends him as a replacement. On the day of the fight, Nagisa convinces everyone on Kazuto's side except Tomoya to eat a combination of Sanae's bread and a jam (similar to Aunt Akiko's from another Key work, ). The result is that everyone including Youhei gets knocked out. With no one else around Tomoya goes forward to fight Sasaki as a substitute.
Sasaki easily gets the upper hand during the first few minutes of the fight. Halfway through Sasaki notices that Tomoya has a bad shoulder and tells him about it, but Tomoya tells the gang leader not to go easy on him and the fight resumes. After knocking Tomoya into the river and landing a couple of powerful blows on him Sasaki thinks the fight is over. Suddenly Tomoya lands a punch on the surprised Sasaki's face and so the fight drags on till night. After fighting fiercely for hours Sasaki and Tomoya rest for a few seconds, before charging and hitting the other in the face; however, Sasaki hits harder causing Tomoya to fall. Just as the fight is about to end, Tomoya gathers all his strength then punches Sasaki hard in the stomach and both fighters fall to the ground. Youhei and Kazuto's gang revives and seeing Tomoya injured rushes forward. The other gang follows suit and when a gang war is about to erupt "Kazuto" arrives causing both sides to stop. Eager to end the fight Sasaki charges at Kazuto and punches him in the face. "Kazuto" is then revealed to be Yukine, crossdressing as her brother. An enraged Tomoya yells at Sasaki for being not understanding Yukine and her wish to rid the city of violence. She cries then tells everyone to stop fighting.
The next day, everyone who was present in the fight is gathered at Kazuto's tombstone while Yukine tearingly talks about her relationship with her brother and how he died. Sasaki is repentant of his actions and to show respect he prays to Kazuto's grave. Both gangs then follow his example. Seeing everybody praying for her brother, Yukine is finally relieved of her grief.
Sasaki makes a cameo appearance in episode 16 of ~After Story~ in the photo on the Christmas card sent to the Okazaki residence.