If i play hidden kennen, , Keeper of Balance and stun a unit but before it resolve my opponent get my kennen back to hand do kennen stun the unit anyway?

Yes — the stun still resolves. Kennen's ability says "Stun a unit," not "a unit here," so once the trigger is on the chain it no longer depends on Kennen's location (FAQ #11954).

The [2] is paid as the trigger finalizes, and the target is locked in at that moment (Rule 352.4.b) — bouncing Kennen with Gust/Star-Crossed removes him but leaves the paid, targeted trigger fully intact. It resolves and stuns the chosen unit; Kennen just ends up in hand and won't be around for the Showdown.

One hidden-specific note: since he was played from Hidden, that stun target had to be chosen from among units at the battlefield where he was hidden (Rule 811.1.d.2) — but that's a targeting restriction at finalization, not something your opponent's bounce can disrupt.