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.
At Naruto's inception, Orochimaru stands as one of the most horrifying villains in the series. From his menacing promises to take over Sasuke's body to his grotesque snake-themed jutsu to the torturous human experimentation he forces on people in pursuit of immortality, Orochimaru is just plain bad.
As the series progresses, though, more and more villains enter the show. Unlike the rest of the villains, Orochimaru doesn't have a compelling backstory explaining his behavior. He becomes farcical. In fact, he's considered so non-threatening that during Boruto, the villain is allowed to attend a parent-teacher conference at Ninja Academy and stand in the same room with people he had mistreated.
He may not try to end the whole world like other villains, but he does kill 60 kids in one go and leave the sole survivor under a pile of corpses. So the fact he morphs into an eccentric grandfather is absurd.
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.