Not very much is known about this shinobi, other than the fact that he is a new member of the Akatsuki, as he took Sasori's position after he was killed by Sakura and Chiyo. He seems to act like a pretty innocent and polite person, and even a little clumsy at times.
His true identity is unknown, as he has yet to have been exposed without his mask yet, and his origination has not yet been revealed. Though it is apparent that he somehow possesses the Sharingan. Leading many to believe he could possibly be Uchiha Obito, due to the fact he only seems to have one eye (Obito gave one of his eyes to Kakashi). However there is no proof or claims of this yet. He did however claim to be Uchiha Madara, in a conversation he had with Pein, but it is unknown if he meant he is Madara in-person, or in some other way.
After joining the Akatsuki, his first assignment was to capture the Three-tailed Turtle jinchuuriki, along with Deidara. The couple managed to restrain the jinchuuriki and transport it back to the Akatsuki's hideout, where Tobi then proceeded to brag about how he defeated the demon with a one-shot knockout technique. Which seemed to be a little bit over-exaggurated.
Him and Deidara later on went on a mission to kill Sasuke, due to the fact that Deidara had always wanted to kill Orochimaru, who had been previously killed by Sasuke. Once they finally met up with him and confronted him, Sasuke made an attempt to cut Tobi in half with his word, after which Tobi got back up, completely undamaged. Tobi was more of a peer in the fight, until being requested to help set up explosives by Deidara. Deidara set off his C4 explosive, which was thought to have killed Tobi in the crossfire. However it is apparent now that he had survived.