Not sure what she is good for...

Halloween Sakura is a good mage-killer. Although her attack is relatively low, her unique, but not preference weapon, Kitty Paddle+ gives effectiveness against mages. She uses it quite well for she has high res and decent attack and speed. The one in the image is +Res -Def, one of her three best IVs by the way. She reaches 37 res at base with that boon, so you can use her with ploys. If you don't want to use her, you can build a Kaze, who is very similar to her, but has higher speed. Though, there are plenty of other daggers that can use it well, not just her and Kaze.

She's an absolute monster against mages and crap against anything else. So, a very specific-use kind of unit, but excellent at what she does.

If I were you, I'd save the one with the best IVs and fodder the other two for her weapon to Kaze and H!Kagero if you have her.

But.... Her obvious use is mage killer. It practically says so on her weapon.

She'll murder mages pretty hard. If you want an extra mage killer you can give her dagger to someone with high atk like Kagero or really any dagger with good res including Kaze.

Also, she comes with dagger valour right? So if you're training up units you can include her on the team and they gain SP faster.

PA Olivia might be especially happy with that weapon because it will compensate for her low atk.

She is THE MAGE-KILLER! And cute as a button.