Saber was added as part of the Fate/Stay Night collaboration in version 3.4 of Honkai: Star Rail. But unfortunately, you can't claim her for free like Archer. Yeah, I'm pretty bummed about that too.

Artoria Pendragon, King of Knights, Saber, has been summoned from the pages of myth for our personal enjoyment. Whether or not you approve of the collaboration between HSR and Fate/Stay Night, it’s ...