I remember reading this short story from a collection of short stories from the same author. They were in Spanish language (Spanish from Spain). The author I think was in Spanish.
In the story a man was walking through a park and I think he got robbed.
Then an alien of light or energy fuses with his body.
The alien protects him from the robber by flashing a bright light (I don't know if the robber dies to the light, but he definitely ends up unconscious).
The light catches the attention of a policeman nearby, who orders the protagonist something (I think it was just to surrender). The alien inside of the protagonist feels this like an agression and flashes the light to the policeman too (again I'm not sure if the policeman ends up dead).
Then the scene jumps to the police agency where the protagonist has just explained what was on the last pages. There, the detectives begin to accuse the man of being either mad or lying, and they decide to begin to be a bit rough, to which the protagonist warns them that the alien may take it as an agression. The alien, again flashes his light and this time around I'm pretty sure the detectives end up dead.
The next scene is of the man (nearby a river? In London?) complaining that the alien doesn't let him stay with anybody, but that he is able to transform energy in anything the protagonist needs. And there is where the story ends.