That girl is on the cover of volume 3, so I guess she's joining the group. By the way, this is why I still think it's stupid that we use the most recent volume cover available by default. This is information I didn't need until the time it's actually relevant. She could've just been some character we'd all wish would stick around, but doesn't. Day is ruined, disappointment immeasurable, yada yada. But showing up on that cover that so far only has included female cast members that are basically harem members? Kinda hard to ignore.
Not super important with a series like this, but still, there are some out there where the volume covers are really a bit too much of a spoiler if you're only starting out on them. It's like, "Oh look, on volume one this guy still has both arms. But in this later volume cover, he seems to have a prosthetic after finally showing up on a cover again. Time to play 'is this where he loses his arm' for a while."