Minor characters the writers are unjustly cruel to

[Kunmui]

Actually, she has aged from 15 to 17 since the show began. But I doubt they'll go any further than that.

 

[Dollar Bill]

Gil Gunderson on The Simpsons.

 

[Randomgirl xx]

But butters actually seems to be getting more respect in the show recently. In 'Super Fun Time', he was basically bossing Cartman around (even if he was unsuccessful) and helped foil the terrorists plan.

Let's not forget AWESOME-O, either.