Johnny was the personality that united the boys more than anyone, probably, because he needed protecting. Even Dally wouldn't say or do anything to hurt Johnny. Ponyboy described Johnny early in the book as a puppy that had been kicked around too much. Johnny took a brutal beating at the hands of the Socs which further exacerbated his vulnerability as he was always nervous and on-edge in a post-traumatic stress sort of way after the beating. Probably most instructive was the incident early in the novel when Johnny told Dally to leave the girls alone at the drive-in (he'd been harassing them in true Dally-style) and Dally looked at him in disbelief, and then said. . .nothing. He just left for the concession stand.
Ponyboy says that line in order to stress to the reader exactly how important Johnny is to the Greaser gang. The line carries more weight than if Ponyboy had said, "Johnny is important to us." Because he said that the gang couldn't function without Johnny, the point is really driven home of exactly how much of an integral part of it Johnny is. If we were talking about a horse and buggy set-up, there is a single pin that links the horse to the trailer. It's commonly referred to as the "kingpin." That's what Johnny is to the Greasers. He is the single entity that links them all together.
What is odd about Johnny's linking force, though, is that he is not a charismatic leader character. He's a follower. In fact, Pony describes him as being like a little puppy that has been kicked around one too many times.
If you can picture a little dark puppy that has been kicked too many times and is lost in a crowd of strangers, you'll have Johnny.
Johnny's home life is awful. He's been beaten to within an inch of his life by the Socs, too. He's a scared and skittish boy, and even Cherry recognizes it right away.
She smiled and her eyes showed that her mind was on something else. "Johnny... he's been hurt bad sometime, hasn't he?" It was more of a statement than a question. "Hurt and scared."
The gang feels an inherent need to protect Johnny at all costs. That, along with their mutual affection and respect for Johnny, are why Johnny has a unifying force about him. It's also why the gang is completely lost following his death.
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