DEIDARA
In the beginning, the two clearly weren't overly close or friendly as Tobi snickers at the idea of Deidara being killed by Uzumaki Naruto; and obviously wouldn't mind taking the ring off Deidara's severed hand, which Deidara protests to. Tobi cheerfully makes the comment that Deidara isn't alright and that the blonde's only weapon is blowing his own art up, a comment that lends him a good strangling from him. They're opposite ends and we can't help but feel that they may never see eye to eye.
Deidara was neutral for the idea that he would be paired with Tobi, but the latter didn't mind at all. Tobi was a fairly positive individual and brought a whole new level to the group that was definatly unique amongst the members. He happily did the missions handed to him and tried to make the best of each situation, cracking jokes along the way. Tobi clearly admired Deidara and constantly refered to him as sempai. It's during this time that Tobi develops a bad habit, one where he annoys Deidara but never on purpose. Deidara is his partner and to accomplish their missions, they must work together as a team.
Throughout their time together, Deidara treats Tobi like his disciple and tries to teach him what it is to be an Akatsuki member. He helps him when he needs it and when he heads off to kill Sasuke, Tobi is not far from him. The blonde takes great care to keep his partner away from harm's way which says a lot about their relationship as a whole. Deidara couldn't have cared less for Tobi but as the days passed and they continued their missions, clealry things changed. Whether the two became friends is a delicate question, but the fact that Deidara dedicated his last moment to apologizing to his partner truly shows what Tobi meant to him.
The two have shown to be a compadible match - maybe more so than the others. When Deidara's explosives go off, Tobi is able to survive and keep on moving without injury. If Tobi gets out of hand, Deidara is the one to keep his feet on the ground. Their personalities clash but balance out in the end - where Deidara is serious and dedicated, Tobi is carefree and goofy. The two make a powerful combination and with Deidara gone - what will become of Tobi?