As ZeroZet said, only way to get cost reduction and skill up for event units aside from their own event are special events held later. So while you CAN get those things up later it is by far more effective to do so during the event. Cost reduction and skill up can really make a character a lot better since you can deploy it earlier and gain more power in total from it.

So if it is a unit you would like to use, then i really recommend trying your best to max her out as much as you can.