Suppose Cilan wasn't the one Striaton Brother chosen to travel with Ash. What if it was Chili? What if Brock and/or Misty weren't chosen as two of eight Kanto Gym Leaders to be Ash's first traveling buddies? What if Sabrina was chosen (or at least considered) or Lt. Surge (maybe not but in the Anime, I wouldn't be very surprised)? And why does it have to be a Gym Leader or one of the female game PCs?
So have fun with this. State your reasons for the character traveling with Ash.