Little Johnny Nunsuch is always at the wrong place at the wrong time. He's present for a lot of the novel's major scenes and the poor kid's usually around to witness some pretty disturbing stuff. First he has to put up with Eustacia, who is like the worst babysitter ever, as she holds him hostage in order to cover up the fact that she's arranging a secret meeting with Damon.
"You like the fire, don't you, Johnny?"
The boy looked up doubtfully at her and murmured, "I don't think I want it any longer."
[...] "Ungrateful little boy, how can you contradict me?" (1.6.35-37)
Then Johnny overhears Eustacia's chat with Damon and sneaks off, only to run into the terrifying reddleman. Terrifying to little Johnny, at least. It's interesting that Johnny is scared of Diggory but not of the heath itself, especially given how connected the reddleman legend is to the culture of the heath. Johnny shows maturity here, in that he's not scared of nature, but also shows how he's still a kid who's easily spooked by ghost stories.
Only unusual sights and sounds frightened the boy. The shrivelled voice of the heath did not alarm him, for that was familiar. (1.8.3)
Johnny is a true child of the heath, and he's more comfortable there than most of the novel's adults. But being a child of the heath also means that he's subject to a lot of the heath's customs and traditions, namely through his somewhat crazy mother, who apparently practices voodoo in her spare time (5.7.34).
The biggest trauma Johnny undergoes is Mrs. Yeobright's death. Though he doesn't actually see her die, he does see her grief-stricken and weak prior to her death, and he has the burden of being the last person to speak to her before she died. And Johnny speaks for pages here.
"And when she saw the young lady look out of the window the old lady knocked again; [...] and then she went away, and walked across to me, and blowed her breath very hard, like this." (5.2.106)
As a result, he becomes an object of fixation for Clym, who's obsessed with finding out about his mother's death. And Johnny also becomes a story-teller here, which is very significant. The locals of the heath are all about their stories – see every scene with Timothy Fairway. Johnny is growing up to become a member of the Egdon chorus, a voice that will carry on the culture and the stories of the heath.
But there may be a problem with this.
Moreover [Johnny] had been ailing again; and Susan now, as ever [...] attributed his indispositions to Eustacia's influence as a witch. (5.2.73)
What's up with Johnny's poor health? Well, we mentioned earlier that he's a true child of the heath, so his poor health may be a symbol for how the world of the heath is going to start dying out as the nineteenth century and the industrial revolution progress. See, Hardy set this novel roughly thirty years in the past. When he was writing it, the countryside had already been largely industrialized and mechanized and traditional ways of life were falling by the wayside in many areas. So Johnny may be Hardy's nod to the future here.