I don't hate many characters. I either think characters are amazing, or they're annoying, or they're just there. And that is because they are all inferior to the one character I absolutely hate:

Tsubaki Sawabe (Your Lie in April)

As a music fanatic, I should have loved Your Lie in April. Instead, Tsubaki completely ruined it for me.

Tsubaki is the secondary female character in Your Lie in April. She is a sports fanatic and is in love with the boy next door, Kousei Arima (no horror references here), who is the main character.

As a child, Kousei was abused by his mother until she passed. Tsubaki was his closest and only friend who knew about this. And what's Tsubaki do? She kicks, hits, smacks, and creates lots of physical pain to Kousei.

I realise that the primary female character, Kaori, also did the same thing. But unlike Tsubaki, Kaori didn't know Kousei's background, so there's a little more slack there. It's too bad he didn't speak up.

The physical pain was meant to be for comedy reasons, but I never found it funny. What made it worse was that Tsubaki's character didn't grow over the show. She continued kicking Kousei and he still is her friend.

Oh. My. God.

And I thought Misa Amane from Death Note was bad.

Your Lie in April would have been better if Tsubaki was more of a background character, like Elizabeth Middleford from Black Butler. Instead, Tsubaki gets about as much screen time as Kaori, who has only a few episodes short of spending all 24 minutes with Kousei.

Such a disappointment for a beautiful anime. Perhaps the manga is better and has more spoken comedy and less physical comedy.

Honorable Mention:
Shou Tucker (Fullmetal Alchemist)

(These two fall more into the annoying category):
Chiho Sasaki (Devil Is A Part-Timer!)

Orion (Amnesia)