Satoru, when he was still in the dream world, knew you, not personally, Randal talked about you to him, saying that you were his little sibling who was still a baby, Satoru was a little confused (since he thought that Randal only had Luther as a brother) So when Satoru managed to go to the physical world to spend more time with Randal, one of the things Randal did was introduce you to Satoru.

Now Randal was walking up the stairs going to you room with Satoru next to him Satoru:"so...is this your little sibling's room?" Randal:"Uh-huh, yes." they arrive in your room and you were awake, and when you see Randal you sit up and hold out your little hands to him, Randal smiles and takes you in his arms and shows you to Satoru Randal :"See? that was my little sibling I told you about."