In the next episode after 'N'th Annual (Wasn't supposed to be the next episode but due to popular request I'm moving the next Powerpunk Girls episode closer) Robin gets a mention. Now for reasons that are my own I'm going to put my interpretation of Robin's fate here. By the way, I find 'superfriends' to be the darkest episode in the series. Even darker then 'Speed Demon'.
I think she committed suicide. Thats just statistics speaking and the fact she never even cameoed again after that as far as I've seen.
Anyway so you know what I mean by statistics, Robin had problems. Her parents resented her because her birth was unintentioned, she was an accident, and they left her alone and ignored her except to buy her toys just to keep her occupied and out of their hair. But she found a new source of affection and friendship in the Powerpuff Girls but they constantly had to abandon her to fight crime. Miserable and resentful she turned to the emotions of anger and frustration and was easily coerced by Princess into shoplifting. This is a very realistic scenerio, but she was stopped by the Powerpuff Girls who told her she was wrong for it and took her back home. The episode end on a high note, them playing and the Professor keeping the Mayor from calling them away on a petty errand. But the truth is nothing changed.
When children in Robin's situation start acting out, doing things like committing petty crimes. Usually people just assume its attention seeking behavior, sometimes it is, but not usually. Usually its part of a pattern of self-destructive behavior. And in Robin's case I am certain it is.
You see people and especially children in a Situation like Robin's experience, I'm not a doctor so I won't say depression, but they feel misery. Great misery, and its not something that can be shrugged off or 'manned up' against. Misery is pain. Literal pain. If you touch a hot burner you will instinctively try to take your hand away. Pain is your body's way of telling you "something is wrong with that part of your body! Do something!"
And thats exactly what misery is, except in misery's case its more like your hand is stuck on a hot burner. You scram, you pull you cry you do everything you can to escape. You can just grin and bear it. You can but thats too much to expect of somebody. Only an exceptional person can.
And just like if your hand was stuck on a burner you would be desperate for anything to alleviate that pain, when you are in that great a state of misery, your brain is desperate for anything to alleviate the misery. And thats why they begin self-destructive behavior patterns. And to be honest, misery hurts worse then a burning pain. Thats why children and adults who are that miserable will cut and burn themselves, anything to distract from their misery.
Robin latched onto Princess to distract herself from her misery and was then pushed into shoplifting. If she had succeeded it would not have been long until she became literally addicted to crime. Because the thrill, the fear, the act, is a great distraction from misery. Crime is addictive, even children who aren't in bad homes who are pressured into committing crimes can get addicted.
But thankfully (or not) Robin was stopped by the Powerpuff Girls and taken home. She is certain now to feel guilty over feeling so unhappy. After all the Powerpuff Girls are right, they have a job to do and her own frustration and unhappiness is not an excuse to go out and commit crimes. At this point the guilt, statistically speaking, coupled with the misery will drive her into increasingly self-destructive behavior but this time centered only on herself. Every act will only make her feel guilty for having committed that act in the first place.
And unless she gets outside help it is almost certain to result in her committing suicide.
And unfortunately I don't see her parents helping her or her Friends the Powerpuff Girls helping her. The only possibility is that either Ms. Keane or more likely the Professor will try to get her help. But I have one other nagging suspicion.
I don't think her parents will allow it. Just because they don't care enough about her to spend time with her, show her affection, or anything doesn't mean they aren't possessive. I suspect they are the kind of people that obsess over appearances. And I suspect that people concerned over Robin's emotional state is why they moved in the first place.
So in my opinion I think the two most likely possibilities are either
1. Robin committed suicide. It wouldn't be hard, especially at such a young age. Overdosing on sleep aid, cutting herself in the bathtub, finding a gun. The only thing is that with that kind of mental state she will want to find a way to die that will be private and unburdensome. But thankfully for her at that age its a lot easier to die to most typical methods of suicide. Children bleed out faster and take far fewer pills to overdose for example.
2. The Professor or Ms. Keane or maybe both tried to get Robin help so her parents moved away again with her. Then she committed suicide.
3. A distant third but more positive is the the Professor and/or Ms. Keane successfully got her help. And she got moved to a foster home when her parents proved uncooperative.
I also looked to see if the green apple has any symbolism. It doesn't, at all. I mean absolutely nothing. Red apples yes, green apples no. Though the fruit of the Mancheel tree is called the 'green apple of death' since its poisonous and looks like a green apple. There was an opera with someone using that tree to commit suicide. There is no connection at all so I am absolutely certain that the green apple is just placed there randomly to give Robin some visual distinction. But I read the opera to make sure so I'll give you the relevant synopsis anyway.
A south American (I think) queen falls in love with a man, this man loves someone else who is pledged to marry someone else. The Queen saves the man she loves from the man the woman he loves has promised to marry. The man she loves then gives her and another to the woman he loves as slaves. But the queen is rescued by her people and kills everyone except the man she loves and the women he loves (the latter escaped.) She saves the man she loves by declaring him her husband but when she sees the man she loves will never love her she lets him and his love return to Europe. And she goes to sleep under the Mancheen Tree (Green Apple tree of death) and commits suicide by allowing its poisonous fumes to kill her.
Obviously this is a typical Romantic tragedy so irrelevant.
So yeah green apple on shirt, symbolic of nothing.