Klee is likely until mid-November
You probably shouldn't go into Genshin Impact with the mentality of collecting every character.
Although the game is quite generous in terms of allowing players to experience the entire campaign without spending a dime, it is a gacha game after all, so getting characters in and out is part of the game's framework. To that end , Venti, one of the first gacha-linked party members, was expelled. Klee, the bomb maniac, intervenes.
Klee is now live under the "Sparking Steps" banner, which is a fancy name for one of Genshin Impact's gacha menu options. You'll need to make "wishes" inside this banner to catch Klee, which is a five-star "jumper" (reward). Thank goodness this game is free and you don't need all those characters to progress; even if the FOMO angle is real.
As you can see in her debut video below, she kicks ass: she features plenty of slow marching away from enemy mobs exploding in her wake. She would make a great addition to my group, but since I don't spend money on gambling; if she doesn't show up, she doesn't show up.
So what's next in the game? Well, we get a big update on November 11 and another on December 23. Klee should cycle to the old date.