Ralph is an unseen character who was talking to Milo on the phone in the movie.
After Milo got home from school, his phone was ringing and it was his friend Ralph. Ralph was talking to Milo until the tollbooth appeared in his room. Milo told Ralph that he would be back, but Milo discovered the lands beyond the tollbooth. After his adventures, Milo realized that Ralph was still on the phone and Milo had only been gone for 5 minutes. He called Ralph crazy but Ralph was telling the truth. Just then, Ralph said there was a box in his room, and it was the tollbooth. Milo hung up.
It is possible that Ralph ended up on the same adventure Milo had been on.