This glitch is still quite a mystery and needs a lot in the way of research as to why it works, but I have discovered a couple of things recently about it.

1. The Pron glitch will work with ANY character that has an empty inventory by the time to talk to the 2nd Councilman at Skyhaven after learning the Word of Power.

2. The game will always decrease stats by quite a lot, normally resulting in an underflow of Speed, Damage, and Defense.

3. The game calculates these stat changes backwards, starting with the person with an empty inventory who is closest to Slot 5 (using Switch does not change this whatsoever). This usually is either Kara, but if you're playing as Aron, this will be Laya. However, if the person in slot 5 has an item but the person in slot 4 doesn't (usually Laya/Gwyn but would be Kara in Aron's quest) then the game will calculate that person first, and so on.

4. The person that gets calculated first will get the heaviest hit from the stat decreases. While I am still trying to figure out what exact number it decreases by (I so far have averaged speed to drop by between 83 and 85, while defense for this person drops by about a thousand), it seems to change from play to play, perhaps even frame by frame. Any other characters affected by the glitch will see drops to their stats by a much smaller amount.

5. The others who get their stats decreased may see an underflow in defense, but it also may not be enough. Their speed seems to drop by about 14 and their defense by 104. This would not make the other characters faster, but providing their natural defense is low enough, it would underflow it. However, beware that leveling in Lashute could overflow the defense value and leave your character very weak.

6. Though glitch this also underflows the Damage stat (by quite a lot usually), for some reason it has no real affect on the damage calculation. I don't know why, nor do I care to know at this point unless it can be done to figure out a way to one-hit Rulakir and Dark Force.

Conclusion:

The lower level you do this at, the better. You could technically get 3 characters (perhaps 4 if you are playing a different quest where you can stack the key items on one character, which won't happen in Aron's quest if you use Gen Skip) but with weaker results for other characters. That being said, I can only really recommend possibly doing more than one character during Adan's quest in an All Gen run because Adan's is by far the fastest 3rd generation to speedrun (even faster than a skipless Aron quest where you played through Nial's quest), therefore getting Gwyn the extra stat buff would be nice as well, providing their levels are low enough (below 35 I would recommend for safety for any other person aside from the one in the last slot). Leveling up can still provide overflow.

However, it is safe to say that you probably want somebody with a strong GiRes as your go-to for this glitch, so most likely Laya (or Kara if her speed is low enough, allowing her to attack first and wipe out the front line during Rulakir's battle). Having a very low speed is not necessarily a bad thing, it just means that you will have to adjust your GiRes spending accordingly. Using the glitch on Wren is pointless, as is with Mieu. It is possible your Main Character could benefit from it if you make him into more of a support role during the final battle (I could only possibly see this benefiting Sean though, but you still have Laya to use it on).