What is the lowest precentage KO possible? [closed]

Almost impossible to tell and it's character specific.

Each stage has a different off-screen kill line. So it's stage specific as well.

The lightest character in the game is Jigglypuff. The strongest attack in the game is a fully charged home run bat attack.

If you use a home run bat against a Jigglypuff while standing on the edge of a almost any stage, it will KO her every time.

The "red critical line" you mention in the question is calculated based on the knockback about to be delivered combined with how far away the blast line in the impending direction is. So if you were to stand right against the blast line on a walk-off stage, pretty much any attack of nonzero knockback will cause both said effect and a KO at any percentage (including negative ones).

Now, given the edit made to the question stating this:

conisder it from the center of the stage (person reciving hit at center, other where ever they must be) and not the edge of it. Also, it would be taking place on an Omega stage, the matchup though would be dependent on getting the best answer.

...the answer is still "far too many variables". What's the weight of the target? Is the attacker at high enough damage that the "rage effect" is amplifying their knockback? What's the angle of the hitbox? Should the target be DI'ing or SDI'ing to surivive, or to die faster? Are items like the Home-Run Bat, Dragoon, or Daybreak allowed? What about the Beetle or Boss Galaga? Do all Omega stages really have the same dimensions? (Some people suggest it's not the case.) Are you playing in Stamina mode and therefore at -150% damage (that's how the engine treats 150HP)?