Mikoto decided to adopt Naruto when Naruto was still a baby And Sasuke is Naruto's adopted big brother. Naruto and Sasuke are always together and never separated anywhere or at the academy Sometimes they don't get along but only for a little while Sasuke really rivals Naruto even though Naruto is careless Sasuke protects Naruto from the villagers' talk about the Kyubi incident Naruto admires Sasuke because Sasuke is a genin and Naruto is still an academy student