Pokemon Journeys will bring back Dawn in a two-part anime arc airing later this year. Ash's friend and traveling companion from the Pokemon Diamond & Pearl era of the Pokemon anime series is making her first appearance in nearly a decade. Dawn will assist Ash, Goh, and Chloe in protecting the Legendary Pokemon Cresselia and the Mythical Pokemon Darkrai from a plot by Team Rocket. The surprise return was announced today in Japan along with a minute-long trailer for the upcoming arc. The trailer shows Dawn mostly interacting with Chloe, but we do see Ash and Dawn's reunion in the trailer as well. Dawn's Piplup is also confirmed to appear in the arc. You can check it out below.
アニポケは毎週金曜よる6時55分～テレビ東京系で放送中— アニメ「ポケットモンスター」公式 (@anipoke_PR) July 2, 2021
Dawn first appeared in Pokemon the Series: Diamond and Pearl as a new traveling companion to Ash and Brock. Dawn was a fledgling Pokemon coordinator and most her major storylines focused on her growth as a trailer. She eventually left Ash to travel to the Hoenn region and made several appearances in Pokemon the Series: Black & White. Dawn hasn't appeared in the Pokemon anime since 2012.
Dawn's appearance in the anime series follows the previous appearances of Iris and Ash's classmates from the Alola region in Pokemon Journeys. Brock and Misty also made several appearances in Pokemon the Series: Sun and Moon. Dawn's appearance is likely tied to the pending release of Pokemon Brilliant Diamond and Shining Pearl, which are remakes of the Pokemon Diamond and Pearl games. Dawn's appearance is largely modelled on the female protagonists from those games.