So that double/triple atk buff of hers isn't small at all; +30% double/+20% triple on two people without some kind of downside.

She's not a foundational level piece that can automatically just be thrown into any ol water team in the first few slots, but she has stuff to work with.
That +25 burst to an ally skill? Natural fit with Shingen.
The combo atk buff on top of that? She's good for speeding up the rate at which you can burst in general (she'd have to go in either 2nd or 3rd slot).
That 50% fire resist buff? Either that can be paired up with someone with a cover ability (like Kushinada-hime) to help tank random hitting overdrives, or just simply mitigate a lot of damage from hit all overdrives for a couple of characters (can stack with damage cuts like Snow Raphael's or Joan's). Granted, the schedule is light on fire events for a while... but it's something to keep in mind in the future, along with whenever fire ragnarok raids get implemented.

All in all, the easy call right now is to treat her as kind of like Michael in a Shingen team: her job, first and foremost, is to guarantee full bursts with Provisional Forest. But you will need your other slots to handle debuffs.