>>51768802I think a mon that is either heavily their polar opposite or a hard counter to them. I vote a Steel/Fire type or Steel/Psychic. So a Heatran or a Metagross. Those would be things that a Gardevoir - in terms of battling - would have to work really hard to overcome the basic advantage they have over them.
In terms of personality- well that would be dictated by the individual Pokemon itself. But I like the idea of a really analytic, binary, and black and white idealized Metagross, who doesn't take the time to really think about people on an emotional or nuanced level - to them you are X or Y, 0s and 1s, nothing more nothing less. They are hard headed to a heavy fault and causes problems for others because of that while the Gardevoir has a incredible amount of empathy for others and takes the time to examine those nuances in people to understand them more, and this causes them to butts heads with this Metagross over its actions towards others. Polar opposite ideals basically.
For a Heatran, just make it evil and cruel. It wants to set the world ablaze - destroying everything in search of power and control like a devil or a demon. Gardevoir would fight against that, even with the heavy disadvantage - like knight!