Vision

Vision may be considered a definitive ally of Tony Stark in Captain America: Civil War, simply because it is an evolved JARVIS. A sentient android, we cannot say that he necessarily needs reasons in order to pick a side.

However, is it really so? As seen in Age of Ultron, Vision is one the side of life "“ and he will be involved in a war/clash which will most likely result in human casualties. And this isn't all, sine these aren't normal humans, but humans who have the potential of saving lives against bigger threats.

The point is that he may not be a double agent "“ instead, he may just simply renounce his allegiance to Iron Man and be neutral during Captain America: Civil War.