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I just watched the first ep of May's DP cameo out of curiosity, the one with the tag battle in the restaurant. May's Japanese voice sounded natural in this episode, I guess whatever effected Kaori didn't happen till after 2008 because she sounded fine in this ep, and she had May do a lot of expressions like excitement or anger over TR stealing the food, etc.
Funnily enough this ep if you count it separately from the Wallace Cup, felt just like the JN cameos. May comes back, tells Dawn and Ash about everything she did, the group battle in a tag battle in the restaurant, TR steals the food and interrupts inbetween, and then May/Dawn finish their battle with the two trainers there later.
What I really liked though was the sense of progression. May traveled through Johto and to Snowpoint alone, her Eevee evolved to Glaceon by touching the ice rock, we see her talk about Max, Drew and Harley, we see her show 3 Johto ribbons proving she won 3 Contests off-screen, she gives the Piplup necklace to Dawn, and she brings out the half ribbon and reunites it with Ash's on the balcony.
It was all very sweet. Seeing May in the Emerald outfit was also a nice touch.