Steff had a crush on Andie.
This is why he bullies her. He's angry she doesn't return his feelings. Andie's mom was severely neglectful.
This is pretty much written into the film. However, we don't know the extent of her neglect. It seems Andie doesn't know much about her own mother, hinting she was never really around at all. Andie's dad was an alcoholic.
We see him drinking in the film. He might use alcohol to cope with the loss of his wife. Andie doesn't confront him on this since she pretty much lost hope of him ever battling his addiction. Steff had a crush on Blaine.
The way he acts is pretty similar to a jealous boyfriend. He doesn't show much affection towards Blaine, throwing insults at him for example. He acts this way to try and cover up his feelings since he's insecure about being gay. (Or bi if he liked both Blaine AND Andie, which is why he treated them both poorly.) Duckie doesn't graduate high school.
He fails/drops out. No matter how much Andie tried to help him bring his grades up, he kept messing around instead. He might get a low-paying job somewhere near the neighborhood. Or he may try to get his GED. On the other hand, Andie gets into a good college on a scholarship.
She worked hard to try and bring herself out of poverty and succeeds.