I'm not sure if it counts, but in the manga of Blue Exorcist, Mephisto's personality just takes a steep drop a lot of times where it seems like he has evil intentions...but then he seems good again. He's like a roller coaster...I truly believe he's mad.

if you're not sure negative character development is when a character starts out as the best or a good/ok/decent human being (villains not included) and starts going downhill, making wrong choices with a 50/50 chance of their arc ending in needing redemption or destroying themselves

Or destroying the world...
But seriously, Mephisto protects Rin and the other protagonists, but he also mentions how they're just pieces in his game of chess...
And then he also sets up dangerous situations to test Rin's strength...
So...yeah...

Asuna

Too many. It's sad to see a character you like end up with an awful development. Especially when it's a main character or even the protagonist.