Just upgraded Yu to E2, who's now my third operator to get that far. And now I'm eyeing the 5* next because I have that instant max device from the last event.
I think Ptilo would benefit from the levels for extra healing and her passive is very useful too. And then there's Waai Fu... I didn't see her on supports for testing, but she should be the opposite of Gravel, where you drop her in to dish out damage. Could be useful, but I can't tell for sure. Thoughts?