I'm from Poland, and I remember that I saw an old anime on television about a boy who found some sort of red crystal that allowed him to transform into/summon red robot. He was attacked by people with similar crystals in different colors, who also could transform into/summon robots. I think they could've used them kind of like stands in JoJo, but I'm not sure. I think that in one episode he was attacked/used his crystal in his room. Also I think I remember him standing on a roof of his school campus with the robot floating near him.
For some unknown reason I associate it with Power Rangers. At this point I'm not even sure if this is real anime, or if I am just imagining things.