Brineborn Cutthroat

The reward here reads backward from how blue usually pays you. Most flash creatures ask you to hold up mana defensively and cash out with a two-for-one on the opponent's turn; this one turns that same reactive posture into a growth engine. Every spell you cast while it isn't your turn feeds a +1/+1 counter, so the deck built around it wants exactly the things a tempo blue deck already does: cheap counters, bounce, instant-speed removal, and a low curve that lets you fire off two or three spells across a single opposing turn. The friction is that none of the reward comes for free on your own turn; the counter only stacks when you're operating in someone else's window, which pushes the pilot toward the disciplined mana-open lines that flash decks reward anyway. Left alone it's a fragile 2/1 that trades with almost anything, but a couple of end-step interactions turn it into a real clock, and because the counters are permanent, the pressure compounds across turns rather than resetting. It's a purebred design for the archetype that wants to say "no" repeatedly and then win with the same cards it was defending itself with, collapsing the old split between a control deck's interaction and its threats into a single two-drop.