Typhlosion130 You've made some good arguments, let me respond to them in order.
First, I wasn't claiming Kirby's soul would match up to Frisk or Chara- they both have strong souls compared even to other humans. However, there is no reason to think Kirby's soul would be weaker than a humans since it exhibits the same properties; in particular, it doesn't instantly disintegrate when exposed. Directly responding to the argument, the speed of the soul doesn't really matter since speed is equalized. I'm unsure what you mean by strength- I presume you mean either attack power (which is irrelevant, since Kirby can deal enough damage physically) or durability/HP (which I address further down).
I agree that Kirby has no special defense for his soul. Even if he did, KR would simply bypass it. The point is that even taking damage from KR won't kill Kirby before he is able to heal it back- remember, KR acts more like a poison than normal damage.
I also agree that Sans has the advantage in speed (I said as much in my first post), but Kirby's speed is still an asset in avoiding attacks. Note that Sans speed is faster than light on his page. Paired with Kirby's FTL reaction time, Kirby will not be easy to hit. Sans' higher speed also is not as much of an asset, since he doesn't use melee attacks. It is already assumed that Kirby won't be able to hit him until he is tired, anyway.
Next, I understand that Kirby has only one shot to win, while Chara/Frisk had infinite tries. However, Kirby also has advantages they did not-
- High HP- Earlier, I mentioned Kirby's HP. Using Undertale as a source, a soul's HP is determined by its LV, which is in turn determined by the number of victims killed. To quote it directly, "[LOVE, too, is an acronym. It stands for "Level of Violence." A way of measuring someone's capacity to hurt. The more you kill, the easier it becomes to distance yourself. The more you distance yourself, the less you will hurt. The more easily you can bring yourself to hurt others.]" This is supported by the game's mechanics- HP increases with LV. KR is also determined by how many he's killed. Thus, HP and KR will be roughly proportional- while KR may be thousands of times worse, his soul's HP will be thousands of times more. This is important because KR acts over time (like poison), so having more HP means more time to heal. Thus, having killed more actually gives Kirby an advantage.
- Healing- Kirby has the ability to heal while Sans is attacking, which Chara could not. With an infinite stomach dimension, Kirby should have lots of food at his disposal; but even if we limit him to 5 Maxim Tomatoes (the highest amount he can carry in Squeak Squad), that still allows him to come back from near defeat five times.
- Abilities- Kirby can use abilities to counter Sans' projectiles, which Chara could not do. He can, to name a few, inhale them, block them with Parasol, freeze them with Ice, reflect them with Mirror, grab them with Whip, counter them with Ninja or ESP, or just knock them away with Super Abilities.
Combined, these three factors increase Kirby's chances of survival, and give him more time to avoid or counter Sans' moves.
While gravity does make it easier for Sans to set up his attacks, it can still be countered with Kirby's Warpstar, or other flight abilities like Jet or Hi-Jump. For the reasons above, Kirby can still survive the attacks and adjust his strategy.
Lastly, while a bloodlusted Sans will open with his strongest attack, this is not that different from his normal battle strategy. I would argue that Sans was essentially bloodlusted in his fight against Chara- they were going to destroy all of existence, after all. Plus, this strategy would just exhaust Sans faster, leaving him open to Kirby's attacks- many of which also have time altering qualities (specifically Crash, Cook, Ultra Sword, and Monster Flame). Kirby will probably resort to these power moves sooner when bloodlusted.
Keep in mind, all this isn't even taking Kirby's copy ability into account- if Kirby gets Sans powers from his projectiles or using Scan, it can quickly turn the tide.
In short, Kirby has the tools to survive KR, which will give him time to adjust to and eventually counter Sans' attack spam. Then its only a matter of time until Sans either tires out or tries to trick Kirby.