I heard that if you beat the elite four and champion without losing a single Pokemon, you are rewarded with a Red Gardevoir by a black belt. However, the wiki states that it is bugged and will revert to normal if used, along with a myriad of other forum posts. I can only find forum posts about this topic as last as September 2018, so I was wondering if it was fixed in that period of time from then to now, and the wiki simply hasn’t been updated yet. I know that 1.2.5 will come and fix multiple bugs in bulk, so if it isn’t fixed then I can only assume it will be in that patch.
I believe Red Gardevoir is indeed still bugged, I’d be cautious with it.