When Cherry explains to Ponyboy why she feels like she cannot visit Johnny in the hospital, Ponyboy does not understand at first, but to Cherry, Bob "wasn't just anyone." He was her boyfriend, and even though he could admittedly be a jerk at times, she genuinely cared about him. Cherry believed that Bob could be really sweet, and this characterization of Bob from matters to the reader, because this is a new perspective on Bob's character. Bob could be sincere and caring. He was a leader at school, the kind of boy that other students looked up to. Bob was charismatic. Even though Cherry befriended Johnny along with Ponyboy at the movies, Johnny's role in killing her former boyfriend makes it too difficult for her to reconnect with him.
Cherry meant Bob wasn't just an ordinary person. He was sweet/kind in some way but also has a dark side. He tend to get drunk due to his parents being too 'nice'. She said Bob was a special person and that Pony doesn't understand what she mean. The reader may think Bob is a bad person but we don't know what he is like on the good side.