Uzumaki Twins
Yin and Yang
Kushina gave birth to identical twins that she and Minato named Naruto and Menma. After their parents sacrifice themselves to seal the Kyuubi into Naruto, the twins grow up as orphans. Despite being identical in every physical way possible, their personalities are complete opposites.
Naruto is loud, boisterous, attention-seeking, and slightly dimwitted but possesses nearly-unlimited stamina and determination. Menma is soft-spoken, spiteful, angry, and very intelligent. Unlike Naruto, he focuses more on the mystical portion of ninjutsu and genjutsu rather than the common ninjutsu attacks and physical taijutsu. Where Naruto seeks to become the Hokage for the acknowledgement and respect of the village, Menma seeks absolute power to rule the world.
When they learn that Naruto is the Kyuubi Jinchuuriki, Menma grows to hate Naruto out of jealousy for the power that he possesses locked away. Like Sasuke, Menma abandons Konoha to gain power, but he goes out on his own rather than try to find a master to kiss up to. Eventually, Tobi comes along and invites him into Akatsuki. Naruto is wreaked with guilt, anger, and even more potent determination to find and bring back both his best friend and his brother, whom he loves dearly and unrequitedly.