Sasuke Uchiha (うちはサスケ, Uchiha Sasuke) is one of the last surviving members of Konohagakure‘s Uchiha clan. After his older brother, Itachi, slaughtered their clan, Sasuke made it his mission in life to avenge them. He is added to Team 7 upon becoming a ninja and, through competition with his rival and best friend, Naruto Uzumaki, Sasuke starts developing his skills. Dissatisfied with his progress, causing him to defect from Konoha so that he can acquire the strength needed to have his revenge. His years of seeking vengeance become increasingly demanding and irrational and isolates him from others, leading him to become an international criminal. After proving instrumental in ending the Fourth Shinobi World War and being redeemed by Naruto, Sasuke decides to return to Konoha and dedicates his life to helping protect the village and its inhabitants.
Sasuke was a happy child, eager to please and be worthy of his family name. This changed after his entire family was killed by Itachi, the person he most admired in the world. He became cold toward others, unmoved by and uninterested in what they do or think of him, choosing instead to keep to himself. He stopped using honourifics like “sensei” or “sama“, something considered rude in the Japanese language. His only goal in life became to take revenge for his family’s deaths and, by extension, acquire the power needed for that revenge, even if it meant acquiring an external power that wasn’t his own.
The pursuit of power, difficult enough on its own, is something that Sasuke repeatedly struggles with. Whenever he meets new people he makes an evaluation of their abilities: those he perceives to be weaker than himself he arrogantly disregards, giving them as little attention as possible so that they won’t hold him back; those he perceives as stronger he fixates over and tries to surpass, especially when he believes he should be their equal. As a member of Team 7 he is repeatedly exposed to Kakashi Hatake‘s philosophy that there is strength in teamwork. For a time, Sasuke accepts this, experiencing it firsthand on several of Team 7’s missions. He even starts becoming attached to his team-mates and starts to forget the vengeance he craved. But when he meets Itachi again in Part I and is summarily defeated, Sasuke decides the team is making him weak and choses to leave them and Konoha entirely, focusing solely on himself.