Ganta Igarashi from Deadman Wonderland put me off from finishing the DW anime series for quite some time (though I ended up dropping it anyway due to my preference for the manga) and, to a lesser extent, Shinji Ikari made it quite difficult for me to finish Evangelion for the longest time.

I know for a fact that Shinji was the reason why my sister dropped Eva and that, in Durarara!!, Code Geass, and Shiki, Seiji, Suzaku, and Tohru kept her from finishing those three for a while due to her hatred for Seiji and especially Suzaku and Tohru's self-loathing becoming cynical in her eyes.

When I was viewing Death Note for the first time (I believe it was when I had gotten around to the mid-point of the series or so), I took a little break due to my BURNING hatred for Light. Really, the amount of hatred I had for his character is unfathomable. I don't mind him anymore, but I still don't care for him. There have been other instances where certain characters impacted my enjoyment of their respective series (negatively, needless to say), but not to the point where I put them off, I don't think.

@Kathy - Yay, I don't have to type my response. xD Ugh, Seiji and Suzaku...

I had to stop watching Evangelion for a while due to Asuka Langley Sohryu.

That's why I never finished Noragami or Seven Deadly Sins.

Sakura from Naruto. I stalled the show because of her.