if i flip kennen keeper of balance from hidden, and i have 1 energy. do i have to pay for stun right away? or can i react to him and tap my darius legend for 1 energy, then when that resolves pay the 2 for stun?

Yes — but not quite the way you're imagining. You don't get to defer the [2]; you pay it the moment Kennen's trigger finalizes onto the chain, and tapping Darius happens during that payment.

Kennen's "you may pay [2] to Stun" is a cost within instructions, so it's treated as the trigger's base cost and must be paid to put the trigger on the chain (Rule 383.3.b). So there's no window where the trigger sits on the chain unpaid while you wait for an opponent's reaction. The good news is you don't need one: Darius's legend ability adds resources with Reaction, and resource-adding abilities finalize and resolve immediately any time a cost must be paid (Rules 429.3, 444.2.c). So as you pay the [2], you exhaust Darius for the 1 energy and combine it with your floating 1.

Two things to confirm: Darius's Legion condition is met because Kennen himself was played this turn (Legion checks Finalized cards, Rule 419.4.b), and if you decline to pay, the trigger is removed from the chain as if it never happened (Rule 383.3.a.2). Opponents get their reaction window only after the trigger is already paid for and on the chain — same as FAQ #11954, where bouncing Kennen afterward still lets the stun resolve.