Ok I have a theory.Its that during the XY and xyz adventures through Kalos it was actually Clemont writing a story for Bonnie.
If we think about it it makes sense.Clemont is very protective of Bonnie and probably wouldn't let her travel around Kalos with him being the age she is so he's probably just reliving his memory of his journey.But you guys ask how did Clemont know about Ash and Serena?
Well,everyone knows that ash is a bit more adult in xy and xyz and that he loves travelling and battling gym leaders so what if the Ash we watch on the TV is actually just a version of Ash from Clemonts imagination.And Serena is real except she's a version that Clemont has created from seeing her on the television.Ash would be based off all the television sightings and clement would get to know how his battles went and how strong he is because he's a gym leader and because of this many of the other gym leaders phoned him up and told him that a strong trainer is coming.And Clemons thinks fine he can fight Clembot but then in one phone call one gym leader mentioned that he wants to face him not his robot Clembot.Anything he doesn't know or if Ash takes a while getting place to place he makes stuff up to Please his little sister and to allow her a bedtime story each night.But Clement is also a fan of pokemon showcases and he always thinks that Serenas the best and because of the challenges he can create characters around her and he can infer that she can see quite well and cook nicely too.When Bonnie asks one night if Ash and Serena ever met before he remembers a summer camp that he went to when he was younger.He decided to wander in the forest and watched the whole confrontation with Ash and Serena and he decides to make it though Serena starts to have a crush on Ash.Clement knows that once Ash reaches the top he won't have any more stories to tell but as Bonnie grows up she starts to watch his matches in more detail and Clement knows that Ash became her role model.He watches eacand every match with her and she comments on the irregularity of his moves and his smart way of winning matches.When she's finally ten years old and ready to leave on the long waited sister brother trip around their region he starts on a new arc but this time Ash has a whole host of pokemon ones that Clement knew about but had to do a little research on.His bedtime stories are now all about their (Ash and Serenas) quests to be in the the grand final masterclass of performing and to be in the final of the pokemon battles,which he tells her has never happened before.As Bonnie grows disinterested in these tales he wracks his brains and comes up with the idea of them fighting a new super evil team who are trying to control the legendary Zygote and how Bonnie herself gets a part of one of the Zygotes to trust her and therefore start to travel with her dedenne and her.When she moves onto a brand new region without her brother,Sinnoh,she meets a styalist called Dawn who tells her what she promises are real life stories about when she travelled with Ash and then begins to tell her how many people know him around the world.Bonnie learns about Mistyand Brock,who seems to have taken him everywhere before retiring and trying something new,May and max and then all the different people who met him along their travels.Bonnie writes home each week so the stories come in an envelope but she loves receiving them each week from her brother.
And then finally she meets him and her..and their little girl who instantly adores Bonnie.When they call her family at the end of the day he tells her one thing and she will never forget it.He tells her never to give up because if her pokemon are still fighting then the battle isn't over yet.
As he tells her this his hand reaches up to pet the graying, old pikachu who sits on his shoulders.
