Kirby Star Allies is Kirby's fastest-selling game to date, and Nintendo plans on keeping the Star Allies ball rolling with loads of post-launch content. Notably, new Dream Friends have become playable in updates, with the previous update adding Marx, Gooey, and the trio of animal friends Rick, Kine, and Coo as playable characters. Dataminers have managed to find not only the second wave of post-launch Dream Friends, but the third as well.
Wave two will add Dark Meta Knight, Daroach, and Adeleine as Dream Friends. Dark Meta Knight, an evil counterpart to Kirby's rival Meta Knight, first appeared in Kirby & the Amazing Mirror. Daroach, leader of the thief guild known as the Squeaks, first appeared in Kirby: Squeak Squad as the main antagonist. Adeleine, one of Kirby's many artistic allies, first appeared in Kirby's Dream Land 3, but she had a major role in Kirby 64: The Crystal Shards. This will be Adeleine's second playable appearance following Kirby 64.
Wave three will bring Susie, Taranza, Magolor, and the Jambastion Mages. Susie first appeared in Kirby: Planet Robobot as the executive assistant of the evil Haltmann Works Company. Taranza, toady of Queen Sectonia, was the secondary antagonist of Kirby: Triple Deluxe. Magolor, captain of the Lor Starcutter, was Kirby's ally-turned-final-boss from Kirby's Return to Dream Land. The Jambastion Mages—Francisca, Flamberge, and Zan Partizanne—all made their debut in Kirby Star Allies as bosses, and they will likely play as one character much like Rick, Kine, and Coo.
As neither wave has been officially announced by Nintendo, the release dates are currently unknown. Wave one launched less than two weeks after Kirby Star Allies' release, meaning that wave two could drop any day now.
Source: Nintendo Everything