When Cait Sith steals the Key of to the Temple of Ancients, he's forced to reveal that he's not the innocent fortune teller from the Golden Saucer he'd appeared to be. In fact, he's a spy.
At that point, it's only known that Cait Sith is a puppet controlled by a Shinra employee. Eventually, the party discovers through process of elimination that Cait Sith is controlled by Reeve, the head of the Urban Department in Shinra.
As the head of the Urban Department in Shinra, Reeve was primarily involved in the development and general running of Midgar. Because of this, he's more concerned about plans relating to possible damage to Midgar and has more of a genuine concern for the safety of its citizens than the other head Shinra employees: Scarlet, Heidegger and Palmer, and even the President himself. He, along with the Mayor of Midgar, were the only ones opposed to the destruction and subsequent collapse of the Sector 7 plate to crush AVALANCHE's secret base.
Perhaps above the physical safety of Midgar, Reeve is concerned about the lives of the people who live there. At one point in the game, he argues via Cait Sith with Barret over AVALANCHE's terrorist attacks on the Mako Reactors. He insists that even though the total loss of lives due to the explosions wasn't high, each life loss shouldn't be overlooked. Reeve's opposition to AVALANCHE was different than everyone else in Shinra: he was opposed to the fact that their methods were violent; that they were willing to destroy lives for the so-called "good of the planet".
Despite his beliefs, Reeve lets the President and the other Shinra employees walk all over him. Even though he on occasion speaks up about what he feels is wrong or should be done differently, it only takes a pathetic jibe from Heidegger, a laugh from Scarlet, and an almost pitying pat on the shoulder from the President to placate him. Ironically, just like he's controlling Cait Sith, Shinra is controlling him.
After journeying with Cloud and the others for some time, Reeve begins to question his involvement with Shinra on a deeper level. When the others find out he's a spy, Reeve wants to continue as a part of their team, in the hopes of alleviating his confusion. Knowing they won't let him back in so easily, he takes drastic measures to ensure his place on the team: he kidnaps Barret's daughter Marlene and uses her as a hostage, even threatening to harm her if Cait Sith is denied from the group.
Eventually, as Reeve continues on the journey as Cait Sith, his opinion of Shinra diminishes to almost nothing. Experiencing the kidnapping and attempted murder of a teammate, Tifa -- so similar to that of his own attempt -- strikes a nerve. Still, Reeve knew that he'd be more help to Cloud and company if he stayed in Shinra, and decided to be a spy for those who needed and deserved it most. Even when the Shinra government crumbles and he is exiled from Shinra, Reeve decides to continue lending his support to Cloud to help save the planet.
