That question was answered yesterday with the announcement that Dawn, Ash's companion throughout the Diamond & Pearl arc of the Pokémon anime, will be returning to the series for two special episodes. The episodes, which go on air in Japan later this month on July 23rd and 30th, feature a story showcasing the Lunar duo legendaries Cresselia and Darkrai.
Ash and Goh travel to Sinnoh after hearing about how people in towns around the region are unable to sleep because of nightmares. They speculate that the incidents may have been caused by Darkrai, who has the power to make others see nightmares. Meanwhile, Chloe and Eevee independently decide to visit Sinnoh after Chloe's family's summer vacation holiday plans were cancelled. Excited about being on their first solo trip, they head to Ererna City to watch a Pokémon Contest, running into Dawn and her Piplup on the way.
Statement from Megumi Toyoguchi - Dawn's Japanese VA said:アイリス、シゲルに続き、今度は「ポケットモンスター ダイヤモンド・パール」よりヒカリとポッチャマがTVアニメ「ポケットモンスター」に登場することになりました！ 約9年ぶりの登場ということで、久々にヒカリを演じられるかドキドキしていましたが、収録中はパートナーのポッチャマとのやり取りがとても楽しく、安心感もあり、幸せなひとときをすごせました。
Following Iris and Shigeru, now it's time for Dawn and Piplup from Pokémon Diamond & Pearl to make their apperance in Pokémon Journeys. It's been around 9 years since I last appeared, so I was quite nervous playing Dawn again after such a long time, but during the recording I really enjoyed interacting with her partner Piplup, and it helped me to feel relaxed and enjoy the time.
To those watching Pokémon for the first time with this series, it's nice to meet you, and I hope you'll fall in love with Dawn, who feelings about Pokémon shine as bright as the sun. Her partner Pokémon Piplup is also extremely cute, so please look forward to the special summer episode.
Dawn's last appearance in the anime came back in 2012, in the Black & White episode Goodbye, Junior Cup—Hello, Adventure!, where she battled Ash and Pikachu to a draw before boarding a flight to Johto to participate in the Wallace Cup Contest. We'll have to wait and see if that hanging plot point will be addressed or not in these special episodes.
Also making her return to the Pokémon anime in these special episodes are Team Rocket member Matori, accompanied by her unit of Team Rocket operatives known as the Matori Matrix. Matori and her crew last appeared in the anime roughly one year ago, in an episode that had been delayed for more than a month before its Japanese airing due to production delays related to the COVID-19 pandemic.
株式会社小学館集英社プロダクション(ShoPro)のプレスリリース（2021年7月2日 20時00分）冒険の舞台はシンオウ地方！[夏のスペシャルエピソード]放送決定！ サトシのかつての旅の仲間、ヒカリとポッチャマが登場！prtimes.jp