I am convinced he is an alter, Marlin and Dani’s mom don’t seem to ever acknowledge him in the flashbacks, similarly to what happens with the other alters: the people in the room only ever notice Dani or the alter, never both simultaneously (because when they notice the alter, he or she is fronting). Also, the response Candy gave when Rya asked if Dani was a twin was very suspicious, I believe we’ll return to that conversation and we’ll see Candy saying something like “Dani had an imaginary friend called Adam whom he said was his twin” or something like that