So, why did you want to go on a journey, Serena?
[reaches into her pocket and pulls out a blue handkerchief, before walking over to Ash] I wanted to give this back to you. It's yours. Ash, remember when we were talking about Professor Oak's Pokemon Summer Camp back when we were younger?
Well, you let me borrow this way back then.
[A flashback is shown, when a younger Serena was lost in the woods...]
Where is everybody? Where did you all go? Augh! [a Poliwag jumps out of a bush scaring her, causing her to fall and hurt her leg] I knew I didn't want to come to camp. I knew it. Mommy! [starts crying]
[Seconds later, someone comes out of the same bushes. It is young Ash.]
Poliwag? Hey, are you alright? Hi, I'm Ash. Who are you? What's wrong?
Don't worry. See this? [draws out a blue handkerchief] This will make you better. [He wraps it around her right leg] All done. Now watch this. Feel better, feel better right away!
Ow! It still hurts! I can't stand up!
[reaches his hand out to Serena] Don't give up until it's over, okay? Come on! [Serena grabs Ash's hand. As he helps her get back on her feet, she falls into him, almost like a hug. Serena starts blushing a bit.] There you go! [snickers a bit] I think we should be getting back to the campsite, okay? Come on. [Ash still holding Serena's hand leads her out of the forest. Serena still staring in awe and still blushing.]
[Note: Did Ash knew, at the time, he had met his future Kalos companion and love interest? No he did not as Serena did not mention her name to him. This is why he does not remember her when the two encountered each other again at Santalune City]