The only logical choice is that it is Luke. Luke is always with R2 in the movies. If Luke is evil I'm going to start a riot.
That's my feeling. From what we understand we dont see Luke until the end of the film. My feeling is he is seeing R2 for the first time after a LONG absense. This also might be where R2 tells him the news of his long time friend? As I mentioned before - a bitter sweet reunion.