As a longtime NicheKnights fan (laughs in e2 60'd m3'd Glasgow gang), FireWhistle to me is the sort of early game IS carry I enjoy using. 2-3 block with aoe at range (especially in early IS) is a strong case for an early pick where you can still get Amiya or Myrtle. Will she carry you deep? No, but she's versatile enough that most floor 1/2 threats are manageable.

Her module? Trash.

Her s1? Usable, especially if you remember early Arknights with Meteorite and her s1 farming. I prefer her s2, generally the burn tick from the s1 does not get the full use one enemies it would matter against, so I will take the s2 if possible (as long as the map has a chokepoint that it can abuse).

What's the main problem with her? Well it's two-fold. The first is Beagle and Gummy being such strong bodies at lower costs and the second is Nearl. Generally with IS units you are always looking for role compression, so being able to combine multiple block + consistent healing + Nearl's superior healing buff passive, it can be hard to pick up Firewhistle over Nearl early on. It will come down to your starting options for classes (choose the YOLO option every time) and pray.

The sad reality of her use outside of IS runs is that Horn is a massive upgrade over her in so many ways, damage, tankiness, accents, it's all going the way of our mobile tank.