Innnteresting. The ultimate is a potential game winner, and interestingly hard for an opponent to predict when it'll go off. The twiddle as a way to power it up is nice - though the owner will keep wanting to use it on an opponents turn. It also works as early mana acceleration which blue usually needs. And on top of that it lets you top deck a second time if you need to save yourself that way. I think this is very good. Luckily, it's also Shock fodder, so it's probably fair.

I rather like this card, but, as it stands, it's strictly better than Undo (albeit, a different card type), which was already a rather strong card.

I'm not sure what the answer to that is. While I was whining to Link about efficient Planeswalkers, I have to admit that the charm of this guy is that he/she's so damned efficient.

Edit: Holy crap! Is that lady in the auto-card's art 'Undoing' two Hippopotamus? That's awesome!

Yeah the Ultimate can be devastating if opponents let it power up too much, and jmg is right it CAN be an Undo the turn you play it if you want to use it that way albeit on a Planeswalker and at Mythic (not that that really means anything) as opposed to common or uncommon as Undo has been.
That was the main decision in setting the starting loyalty to Two, makes it a very vulnerable Planeswalker.

As to flavour, this guy/gal (not quite sure yet) is one of the Selachi Merfolk who Sparks during the set's storyline and has a major role to play in the backstory.

That first ability makes a bit uncomfortable - you're likely going to be using it to untap lands most of the time. Mana acceleration shouldn't be blue's forte anymore. The way WotC circumvented this issue recently was making the untapping a delayed trigger (Teferi, Hero of Dominaria). That limits considerable how it can be used as a traditional acceleration (ie. as a mana dork), but it still extremely useful when it comes to keeping up counterspell mana and such.

The way I've personally played with this concept is by having blue cards that return lands to your hand. It doesn't exactly accelerate, unless you are missing your land drop of the turn; Singer of the Shore & Godhead of Fantasy. Interestingly, it can also be used to fix your mana in a pinch. Btw, untapping permanents I reserved for the simic walker: Godhead of Medium.

Returning lands to your hand might not seem not that useful at first glance, but anyone who has played with Quirion Ranger knows how powerful the effect is. Ranger would still be good even if it didn't untap anything - that's a mere bonus there. Then we have straight-up broken stuff like Gush.

As it was stated, this was already a better Undo. Now the last ability alone is much, much stronger since it can return +1 targets to hands and can target permanents other than creatures. Was this boost really necessary?

[+1] Twiddle, [0] Frobble, [-1] Unsummon.

This is the bluest planeswalker ever.

@Vitenka: What's "frobble" referring to?

Draw a card, discard a card.

Loot. Literally everyone else calls that loot

Yes; lkooting is a form of frobbling. Generically, frobbling is "Manipulating stuff".

Well, every effect is frobbling, then

You're just not grokking frobbling, maaan :)