Bad guy likes good girl. Good girl falls in love with bad guy. Good girl becomes bad girl. Now that bad girl dumps bad guy and begin to like good boy. Good boy falls in love with bad girl. Good boy becomes bad guy. Bad guy dumps bad girl and begin to like good girl. Series continues without any stable relationship.  

You might be right, albeit in a pessimistic mindset!

What are the definitions of Good girl and bas gril?

Brother in this world there two kinds of girls are exist. One is Good girl and other is Very good girl. Therefore there is no bad girl exist. All guys are bad or very bad. A girl Slept with a guy that doesn't mean girls became bad. Those girls are bad who scare with this entire society. Those girls are who want to sleep with guy but for the shake of society supressing their desire. You know such type of personlity called Hypocrite.

Definition of good and bad stands on the morality and character. It applies to both boys and girls. Those who loose their own moral values and harm their own self respect are considered bad. Society can’t judge the person better than himself/herself. Having sex with someone doesn’t make one bad. Those are considered bad who harm their own conscience for mere gratification.  Those who use others badly for their mere benefits are bad. Those who lost their self-respect, moral values and ethics are harmful to others and they are bad.

What is the barometer of Morality and character. How do you know who has good morality and good character.

Nobody can judge one’s morality and character better than himself/herself. The origin for it is don’t hurt anyone including yourself intentionally. If you hurt, you are downgrading yourself.