Just because an anime character starts out great doesn't mean they stay that way throughout the whole series. In fact, there are plenty of anime characters who got worse over time.
In some cases, these characters just didn't get the greatest development; they aren't terrible, but they could stand for some improvement. This is true of Elias Ainsworth from The Ancient Magus' Bride. Bone Daddy isn't a horrible character, but he does some things that make his relationship with Chise seriously questionable. In other cases, the character is wrecked beyond all redemption. For example, does anybody really still like Gohan after the Cell Games? And what Sword Art Online fan isn't righteously pissed when Asuna spends all of Season 2 locked in a cage?
It's frustrating when beloved characters get less awesome over the course of the show. Let's commiserate!
Remember when Gohan beats the living tar out of Vegeta by transforming into a Great Ape? At a very young age, Gohan takes on enemies who are far more powerful than him despite insurmountable odds. Is he scared? Of course, but despite his fear, he does what's necessary to save humanity.
That all falls apart as the series progresses. During the Cell Games, Gohan gets too scared to fight. This change seems less like reasonable character development and more a means of creating conflict. Later, he stops training altogether for seven whole years, though he knows new opponents threaten Earth's safety.
He comes back with such little fighting ability he makes Yamcha look like a champion.
Ash doesn't actually change much over the course of the series; he stays the same happy-go-lucky guy who knows almost nothing about Pokémon. At the start of his journey, this is completely understandable. However, as the trainer works with more rare species, he seems to become less knowledgeable. When he releases Butterfree, Ash understands the Pokémon wants to stay behind and mate. But when the same thing happens with a Cottonee in later episodes, Ash is totally clueless.
If you are a fan who tracks Ash throughout his various journeys, you may find his lack of development hard to forgive.
When Light Yagami first gets his hands on the Death Note, he genuinely wants to create a better world. He kills criminals without checking if they're actually guilty, but he still means well. As the series progresses, the character's focus shifts from bettering humanity to defeating L, avoiding getting caught, and amassing power. He repeatedly compromises his own morals, killing people solely because they get in his way. Light Yagami even puts his own family in danger.
Power corrupts him, but when he actually comes face to face with death, the marauder becomes truly pathetic. He screams, cries, and begs for his life, drooling as he brays a litany of excuses.
When Asuna first appears, she's an awesome heroine who holds her own in battle, but she eventually accepts Kirito as an equal partner. That's why their eventual romantic relationship is worth supporting. In the second season of Sword Art Online, Asuna falls victim to a creep named Nobuyuki Sugou. He slobbers all over her, locks her in a cage, and renders her helpless. Asuna relies almost completely on Kirito to rescue her.
A formerly capable fighter morphs into a damsel in distress; the change is surprising and unwelcome.