Obito Uchiha appears in the anime in chapter 239 as a young child in the Hidden Leaf Village during the Third Great Ninja War. Although viewers see him in "Naruto" as Kakashi's teammate, his real backstory emerges in the sequel. Here's how the unassuming boy from Kakashi's team went on to have such an impact on the series, and everything else you needed to know about Obito Uchiha.
In several ways, Obito Uchiha serves as a foil to Naruto, particularly in the early lives they led. As a child, Obito was unaware of his parents' identity and only had his grandmother to take care of him. In the academy, he formed a one-sided rivalry with Kakashi Hatake, who was leagues above him. He also became close friends with Rin Nohara, and later fell in love with her, but his feelings were never reciprocated. It's safe to say that Obito didn't exactly love his childhood.
You'd think a massive boulder crushing a child would be enough to finish him off, but Obito is rescued and helped back to his feet by an elderly Madara Uchiha, with the assistance of White Zetsu. In the shinobi world, it's not exactly out of pure kindness that someone like Madara chooses to save a child. Instead, he had a much darker reason to keep Obito Uchiha alive. After White Zetsu is informed that Kakashi and Rin are to be killed by Kirigakure ninjas, Obito rushes out, thanking Madara for his help. Obito arrives at the scene in time to watch Kakashi pierce Rin's heart with a Chidori, killing her while being surrounded by Kirigakure ninjas. Kakashi collapses after the act and Obito goes on a rampage, annihilating every Kirigakure ninja. The Uchiha returns to Madara and begs for a world where Rin, Kakashi and he can be together. Madara teaches him about his Mugen Tsukuyomi plan and everything he needs to move forward. Having entrusted Obito with all the knowledge, Madara disconnects his life support and tells Obito to act as Madara Uchiha, until his return.
It isn't easy to put a plan into motion that involves capturing the strongest beasts the world has ever seen, and it is even less so when you do it under the guise of achieving "peace." The first part of Obito's plan was deceiving the fledgling Akatsuki and having its members killed, to get Nagato to side with him. With Nagato and his Rinnegan on his side, Obito started hunting the rest of the tailed beasts. Meanwhile, he took up the nickname of Tobi and adopted a goofy, amusing personality. Of course, in a world so brutal, it's nice to at least have some masked guy providing a respite of temporary comic relief. Twelve years before the start of the series, Obito launched his plan of attacking Konoha, unleashing the Nine-tails by extracting it from Kushina Uzumaki during childbirth. Kushina's husband (and Fourth Hokage) Minato Namikaze fails to recognize Obito, yet nevertheless succeeds in defeating him. Itachi was tasked with massacring the Uchiha clan to avoid the coup d'etat years later. Obito, under the guise of Madara, was once again instrumental in an act of violence, as he aided Itachi in slaughtering the Konoha Military Police Force — and subsequently offered Itachi a position in the Akatsuki.