Knowledge

She knew how to make him laugh. It was almost effortless with a jolly kid like him.

She also knew how to make him smile. The simplest of gestures like sharing food (albeit none she made), challenging him to a pokebattle, and her probably bad impression of Brock, which she only performed in her silliest moments, made the curve of joy appear on his face. His dimples only appeared when his grin was 3/4's wide. They stayed hidden in their cheek caverns for smaller smiles. This was the stuff only best friends knew.

"Oh come on, Ash don't tell me you actually thought Groundeon was real!? There are no known ground eevelutions yet, plus that is the MOST unimaginative name ever." She used her most deadpan look because it annoyed him more than her smirk.

He gave her his indignant how-did-you-know-and-i-didn't pout.

A pause before the killing line- "I know you watched those Pokemon conspiracy videos again! Ketchum, stop believing these guys on the pokewebs who say they can predict new species and evolutions. Why don't you catch the ones you actually know about?"

"Ugh! Quit nagging me, Misty!" He stalked off because he knew she was right as usual.

Of course, most importantly she knew how to irk him too!