A continuation of this post:
So these screenshots are taken from one of the videos circulating around YouTube with L's battle theme -- Pokeli's, specifically. I'd have to play through the game all over again to get them off my Switch, so these will have to do.
This post will get long because of all the screenshots, so this is all going under the cut!
To start, a comparison:
[image: L is looking at his Mega Ring, his blinding white eye visible. The Keystone is dark, making the internal symbol of Mega Evolution difficult to see]
[image: Tarragon's starting battle pose. The Keystone in his helmet is bright]
Tarragon's Keystone is the normal colors; L's is not. And this isn't because of ambient lighting, I bet -- the fight with L happens outside, sure, but the player character's keystone is always bright no matter the time of day or whether we're inside or outside. For example -- the Tarragon screenshot is when you fight him before your Canari rank fight, so we're indoors for this encounter.
Something interesting, too -- while Tarragon and most of the other Trainers have their Keystones start to glow as soon as they touch them, L's starts glowing when he lets go and tosses the ring.
[image desc: three screenshots in quick succession; L tosses his Mega Ring into the air and it gains a halo of light; the Keystone starts glowing so brightly that the ring cannot be seen. The glow mostly fades by the time L reaches out to catch the ring in the third image]
I'd have to go back and look, but I don't know if anyone else's Keystone gets that halo effect in the first image. And this post is getting long, so you'll have to forgive me for not including them here.
The second image is what the other Trainers' Keystones start looking like almost as soon as they're activated. It's why Urbain/Taunie winces when the Keystone activates -- keeping that thing on a bracelet so close to your face has got to be like when a camera flash goes off. I'd need to take more screenshots to compare L's Keystone to anyone else's, because while the glow is bright enough to overtake the Keystones of others, I don't think it reaches L's level of brightness.
And then --
[image desc: two screenshots taken in quick succession. In the first, L is holding his Mega Ring in his fist; the glow is leaking out through his fingers. In the second, he is holding his fist out towards his Gyarados, the glow of the Keystone bright enough to explode from his hand despite hiding the Keystone in his grip.]
No one else hides their Keystone when it activates. All their Keystones reach the same level of brightness that L's has when he's keeping it clenched in his fist. Either this was a default setting for all Mega Evolutions and they just applied the light animation where they needed it...or L's Keystone has some extra punch that others don't. Again, because he was exposed to the ultimate weapon and what have you.
So I'm stuck with wondering -- is it possible that other attempts to Mega Evolve turned into a flashbang in L's face, and he had to learn to hide the Keystone to avoid accidentally blinding his working eye...or is this the first time he's actually tried Mega Evolving after the ultimate weapon, and the Keystone is waking up properly after sitting in his pocket unused for five years?
I'd need more data to say which one it is conclusively (or, y'know, just let my brain run wild), but considering he was lacking in memory for a while, I wouldn't be surprised if it's the latter, and he just held onto the Mega Ring because he knew it was important but couldn't remember why.