I like her as white-blue, but I feel that green-blue could also make sense.

I think I'd prefer to see this as a 2/1 for 2 than a 3/1 for 3. They're both fairly unsolvable, obviously, but the 3-damage-a-turn clock is pretty nasty.

I do like the last line (prevent combat damage) for when you have to deviate from the main plan enough to use her as a blocker.

yeah a 2/1 for 2 sounds better here,3/1 is just too strong even at Rare.

Point taken. :)

You know, she might need to be 1/1... That, or have Frost Titan hexproof instead of real hexproof.

nah non targeted removal can still deal with her, Wrath of God sorry Day of Judgment or a well timed Diabolic Edict for example

If you're thinking EDH general then this is certainly a good creature, but nothing that Sygg, River Guide-fueled Tron decks couldn't already do. If you're designing this for EDH I think it would be really great to go up to 3 CMC and pick up another activated ability of some sort instead of preventing the damage (since combined with Hexproof this thing is a tank) to add some variety into the general.

She wasn't actually conceived for EDH, which is why she doesn't have many tricks up her sleeve.