It's funny that we don't see a lot of the big bosses in this book. Jeanine shows up once to be both logical and evil ("I want to find out what happens when someone drowns, so I'll drown this Tris girl"); and Max shows up in his role of leader of the Dauntless to watch the initiates go through their fear landscapes (which is what you do if you don't have TV.)
But other than that, we don't see him much at all. We do hear that he's made some bad changes to Dauntless. For instance, he made the initiation ritual more competitive and brutal (18.108). And though he's not a direct threat to Tris (not the way Peter and Eric are), that doesn't mean he isn't dangerous, since he's working willingly with Jeanine on destroying Abnegation.
We don't see a lot of Marcus, Four's abusive dad. In fact, the first time we see him, it's in Four's fear landscape, where Tris protects Four from him. And then when we see him in person, it's not much better: Tris notices how cold his eyes are (37.49), which seems like a giant neon sign saying, "this guy is bad news." We're sure the second book is full of father-son arguments and/or games of catch, but Marcus's main job here is to give Four a reason to join Dauntless.
But Marcus isn't just the bad dad; he's also one of the leaders of Abnegation. Unlike Erudite, which has one leader, Abnegation is led by a ruling council, and we don't have proof that all of the councilors are bad parents. But it is curious to note how almost every leader that we see—with the possible exception of Tris's dad—is also a jerk. Like, we're not sure what we would say if you gave us this choice: would we rather be led by Jeanine (planning a massacre), Max (likes cruelty), or Marcus (the abusive dad)? Are there any other options? Maybe Amity and Candor have better bosses, but we're not betting on it.
The most important Dauntless initiates who started out in other factions have already been covered—Tris, Christina, Will, Al, Peter, Molly, and Drew. That leaves only Edward and Myra, who aren't all that interesting (sorry, you two). Edward started out as Erudite, but he's been training to fight since he was ten (14.37), which explains why he's such a good fighter. In fact, he's the best fighter of the transfers. But that doesn't help him when Peter (or one of his minions) stabs Edward in the eye during the night (16.101). This is kind of a lesson to Tris: her enemies mean business and it's dangerous to stand out. Or, you know, sleep with your eyes closed.
Myra might be another lesson for Tris: she's the worst fighter, she's dating Edward, and when he quits Dauntless, she goes, too. Is this a lesson to Tris not to define herself by the boy she's dating? Or is it a lesson not to be the worst fighter in the group? Whatever way you look at it, Tris doesn't want to be like Myra.
Since the Dauntless-born initiates do most of their training apart from the transfers, we don't see a lot of them. We get some very brief glimpses of some of them, like Rita.
We spend some more time with a few of the other Dauntless-born initiates, either at the paintball game (12) or when Tris goes zip lining (17) or when they're just hanging out, shooting muffins off heads (21). There's Lynn, who isn't very nice to Tris at first; Marlene, who is nicer to Tris; and Uriah, who is extra nice to Tris. When he finds her feeling blue after Edward's been stabbed in the face, he invites her along for the zip lining, even though that's not usually open to transfers (17.14). But even with Uriah, Tris notes that the Dauntless-born initiates are mostly friendly to her when she seems strong, unlike Christina and Will, who had her back when she was weak (23.55). This seems to be a danger with Dauntless folks: they might not help you when you're down.
Most of the people we meet in this book are initiates who are going through what Tris is going through; but we do meet a couple of people who are already in Dauntless, who get to show us how great it is. To which we have to say: are you sure it's worth all the trouble, Tris?
For example, Uriah's older brother Zeke is already in Dauntless, and he takes the kids out for a zip lining field trip. Zeke and Uriah play-fight and tease each other like brothers, which is a nice reminder that family's important, even in Dauntless. But do you really have to fight a bunch of people to get back to your family, Tris?
And then what about Shauna and Lauren—are their lives so great? Lauren teaches the Dauntless-born initiates, although part of her training involves running the new kids through her fear landscape. Which is super personal; like imagine if your teacher just said, "hey, everyone, today we're going to talk about my most personal fears. Yay." Thanks, but we don't need other people seeing our deepest darkest anxieties.
Meanwhile, Shauna's main role here is to tell Tris that Four is a good guy—they were initiates together and Four helped Shauna train when she was no good at fighting. That's really all: she's just a source of info to tell Tris that Four is nice (17.41), and then she disappears. Hello, plot device.
The saddest sack in the bunch is Tori: she's very nice, but where does that get her? She administers the aptitude test for Tris and fakes her results. They meet again in the Dauntless HQ where Tori is a tattoo artist and information source. Besides giving Tris her tattoos, Tori also tells her what happened to her brother Georgie. It's a sad story (cue the weepy music)—or possibly a thriller (cue the dun dun duns). Georgie was Divergent like Tris is, and when the Dauntless boss's found that out, they offed him and made it look like a suicide (20.59). That's terrible for Tori and terrifying for Tris.
Susan and Robert Black are neighbors to the Prior family. Susan flirts (as much as Abnegation ever flirt) with Caleb; and Tris imagines the two of them getting together, and her marrying Robert. And then everyone would live happily and Abnegation-ly ever after—until the Dauntless army comes to kill them all. So, maybe it's a good thing that Robert joins Amity, Tris joins Dauntless, and Caleb joins Erudite. Susan ends up in Abnegation, but the last time we see her, she's safe in the, uh, safehouse (36).
But really, Susan and Robert are mostly useful to us readers (and we're all that matter, right?) as examples of Abnegation "friendship": they're neighbors and they chat. But that's about it. They're not as close with Tris as her Dauntless friends.
Our favorite, #1 minor character doesn't even have a name: it's the nameless factionless man who Tris meets in chapter 3. When she's walking home after her aptitude test, wondering which faction she should choose, she meets a guy who clearly did not choose correctly. He's one of the factionless people, someone who "failed to complete initiation into whatever faction" he chose (3.39). He wears ragged clothes, has a raspy voice, and a gap between his teeth. Like most of these minor characters, he offers a lesson to Tris; and he's nice enough to say his lesson very clearly: "Choose wisely, little girl" (3.53).
But there's more. Like with many of the characters, we learn a bit about Tris from watching her interact with the factionless man: he needs help, but he's a little scary. So though she offers help (as a good Abnegation would), she also thinks about beating him up if he gets violent (as a good Dauntless would). So while she's making her decision to join Abnegation or Dauntless, we see that she might fit in with either faction.