I'm sorry but this far, far exceeds maximum acceptable jank levels. This is too polarizing and too much of a removal from normal combat. It'd make it so Pichus didn't really fight, they just wait till their opponent has nearly killed them and then try to OHKO. Like, would it be powerful enough that Pichus would want to be at a higher percent? So they'd just camp the ledge with fairs till they were at 100%+ and then run in, either about to land a OHKO or get KOd from any tilt? It means their opponent's stock would end when their own stock gets near its end due to the power of its moves, and then the opponent would respawn still ready to die because the Pichu was at a high percent. It adds second meaning to "you're almost dead" which shouldn't be there. There shouldn't ever be positive rewards for being nearly dead; changing the rules up so that there are is redefining how fighting works and that just ruins everything a lot. I'm opposed to there being anything so jank that you take damage and say "Alright, closer to winning!" And regardless of who dealt the damage (Pichu, or its opponent), this is still rewarding the player for chipping away at their own health: the reason the PMBR removed it from Lucario.
There's other reasons that aura is bad, though. Because damage factors into the equation for knockback, having variable damage from an aura status would make it so combos didn't consistently work across Pichu's percents. It's like how when you get hit by Olimar and you don't know how you'll fly because there are 6 different hitboxes that could come out. Also, it's best to avoid any extreme changes; I was trying to keep the Pichu that I made as close to Melee's Pichu as possible while fixing any weaknesses that got in the way of what should be strengths. It would be an extreme character with its speed and early deaths, but aura on top of recoil damage is making it so it would hardly be a fighter at all, more of just a dice roll to see who gets the first hit in after Pichu is at 100%. You'd still have to fix this character even if you tacked the aura and high-percent OHKO move on; most of Pichu's moves just don't work due to having insanely low base knockbacks, high knockback growths, low damage, and garbage range. It deserves a lot more care than "When I'm about to die, you'll die with me."