If you're an anime fan, you've probably at least thought about what your favorite characters are like in bed. While we'd all like to imagine that our favorites are sexual deviants, the truth is that there are some anime characters have just got to be terrible at sex. Yes, even some of the hot ones. Being attractive is one of Light Yagami's main character traits, but he doesn't care whether or not Misa gets off. Some anime characters who would be bad at sex are just straight up selfish, whereas others are literal puppets who do not posses genitals.
While there's probably a deluge of convincing fanfiction that proves these characters are rockstars in the bedroom, die-hard fans can convince themselves of anything. A more critical eye reveals that some anime characters just aren't up to snuff in the boot-knocking department, and there's plenty of evidence from the series to support these claims.
It doesn't matter how much you might like Goku of Dragon Ball Z, the dude has got to be abysmal at banging. While he's obviously good enough at it to make babies, it's unlikely that he's routinely rocking Chi Chi's world. He consistently demonstrates a complete lack of understanding for human limitations, so he'd likely go much too hard without realizing it. Also, he once said that he doesn't kiss his wife, because "that's gross." If he thinks kissing is gross, he's probably not all that interested in moving past first base. If you've ever wondered why Chi Chi's so mad at him all the time, this is why.
While Light Yagami of Death Note is totally capable of turning on the charm, he's not going to do so if he doesn't have to. Light strings along two girlfriends, Misa Amane and Kiyomi Takada, and he doesn't have any real romantic or sexual interest in either of them. While he probably does sleep with them on a regular basis, it's most likely obligatory, passionless sex that he only bothers with to stoke their delusions and to keep them obedient.
One could argue that Light does use dating as a distraction from his life as Kira, but considering how outlandishly selfish he is, and how little he cares about either Misa or Takada as human beings, you can bet that he isn't especially focused on making either of them feel good.
Sasori of Naruto isn't made of human flesh; if you look closely, you might notice that he's actually a wooden puppet. He probably doesn't have genitals, and if he does, they're probably made of wood too. Any kind of sexual activity with Sasori is likely to involve splinters, and if you somehow manage to escape that, you'll get poisoned. Sasori constantly carries around a deadly heavy metal poison that guaruntees death within three days. If you've always wanted to screw a puppet, Sasori is a terrible choice (even when he's in his "pretty" form).
Caster of Fate/Zero is a profoundly disturbing character. He's the servant of a ruthless serial killer, but his evil thoughts extend beyond the range of his job. After his old boss, Joan of Arc, was killed off for being a heretic, Caster decided that he no longer cared about human decency. Before long, he was sexually assaulting and murdering hundreds of young boys, which led up to his current job as an assassin.
It's not just Caster's internal qualities that make him such an unappetizing lay, he's also pretty physically gross. Despite being relatively young looking, Caster has creepy bulging eyes, and his skin is disgustingly oily. When he's not moving, it's easy to mistake him for a freshly-murdered corpse.
Caster is probably the type of dude who tells you whatever you want to hear to get into your pants, then cheats on you when you're at your absolute lowest. Considering how much he enjoys giving his victims false hope, it's pretty obvious that he'd also be a manipulative lover.