Perhaps a better way for you (the student) to answer this question is through the use of a graphic organizer, like perhaps a Venn Diagram, which would allow you to adequately compare and contrast each pair of characters better than in long-hand written format.

Johnny and Bob, for instance, are very different because Johnny is a greaser and Bob is a soc. Both have their troubles, but those troubles are very different -- Bob's problems come from a life of affluence, while Johnny's troubles are related to a life of negligence.

Dally and Johnny may at first seem similar, as they are both greasers, but upon closer inspection, we see that Dally is indeed a true criminal, whereas Johnny is just a small-time hood who is trying to fit in, despite his apparent sensitivity.

Ponyboy and Johnny are fairly similar as well, as both are greasers, but in this case, Johnny does not see the world as Pony does -- he even tells Ponyboy that he never paid attention to sunsets before Ponyboy pointed them out. Both boys are fairly sensitive in comparison to their greaser peers, but Ponyboy's keen sense of art and literature is greater than Johnny's.

Johnny's major characteristic is that he is a very caring and sensitive person.  In that way, he is very different from Bob and Dally.

Bob and Dally are both tough guy types and Johnny certainly is not.

This characteristic of Johnny's is quite similar to Pony's personality.  Both of these younger boys are much more in tune with the ways that other people feel than the older boys in the gang are.

The only thing that I really see in common between Johnny and Dally and Bob is that none of them have normal parents.  Dally has none, Johnny's hate him and Bob's don't care enough about him to try to control him at all.