Alien Abduction: Engineers his own voluntary one at the end of the first book on the basis that, despite some harmless brainwashing, the alien teacher was only observing humanity. There are also hints of I Just Want to Be Special being in play.
Berserk Button: Don't ever insult Hoo-Lan in front of Peter. Not even if you're on the galactic ruling council.
Brain in a Jar: Voluntarily becomes this so the aliens can study how the human brain works. Averts And I Must Scream by way of the jar being able to transmit the brain's neural impulses to Peter's body, allowing for remote piloting. He admits the whole thing would be fascinating if it wasn't his brain he was looking at.
Cloud Cuckoolander: This is a guy who voluntarily allows aliens to kidnap him.
Face–Heel Turn: Subverted — while he helps Broxholm (whom everyone thinks of as evil simply because he's an alien who wants to kidnap kids) escape the school after being exposed, it turns out the aliens he joined aren't really bad.
The Hero: Of the last two books, and ultimately of the whole series.
Meaningful Rename: After leaving Earth and joining the aliens, Hoo-Lan renames him "Krepta", meaning "child of the stars". He still thinks of himself more as Peter Thompson though.
Missing Mom: She abandoned her family when "someone better came along", and his father lost his mom when he was "too old to be adopted", as he put it.
Naked Freakout: When he's brought aboard the New Jersey, Peter goes through a decontamination process that requires The Nudifier to make him strip out of his earth clothes. Being the nerd, Peter isn't even comfortable changing in gym, to say nothing of in the presence of strange beings from another planet. Not helped at all by the fact that, being the first actual human all but three of the aliens aboard the ship have ever even seen, he draws a very big crowd in the observation room.
Rebuilt Pedestal: Has a terrible relationship with his father, and is convinced that he won't even be missed when he decides to go with the aliens. Later on he learns that not only was his father devastated to lose him, but that he dedicated his life to helping other children as penance for his failings as a father. He and Peter make up as a result.
Smart People Wear Glasses: He's both smart and wears glasses, until dropping the latter trait when the aliens casually fix his eyes as part of a routine checkup.
Susan Simmons
The Ace: Peter suggests that she may actually qualify as the "best" student in the class, depending on what criteria Broxholm is using. Her grades aren't as good as Peter's, but they are still excellent. She may not be as angelic as Stacy or Mike, but she's still very well behaved. And she's popular, athletic, and participates in many activities.
Go-Getter Girl: Peter warns that Broxholm might pick her as the "best" student since she's smart, popular, gets good grades, and is overall very well-rounded.
Snooping Little Kid: How she finds out Broxholm's secret in the first place — she followed him home to swipe a nasty note from his briefcase, which he'd left sitting on his front porch, but when she heard a screeching noise inside the house she couldn't help but investigate and so discovered his secret.
Stay with the Aliens: The series ends with her and Duncan joining the aliens in traveling the universe.
Duncan Dougal
Abusive Parents: Mr. Dougal's philosophy is that life is rough, so you might as well get used to it. It's basically a thinly-disguised excuse to physically and emotionally bully both of his sons.
Big Brother Bully: Like their father, Duncan's brother Patrick pretty much uses him as a physical and emotional punching bag.
Blessed with Suck: After he fries his brain one too many times, he begins to hear radio stations in his head, which prevents him from getting any sleep.
Brilliant, but Lazy: Broxholm points out that he was never stupid; he just had tons of untapped potential that the brain fryer unlocked, as do all humans.
Brought Down to Normal: Not during the events of the series, but it's confirmed that the effect of the brain fryer will eventually wear off, leaving him mostly back to his old self but with a unique awareness of just how much his brain is capable of.
The Bully: Until the second book, where he tries to turn over a new leaf.
Butt-Monkey: He can NOT catch a break at all in the second book.
Freudian Excuse: He's pretty much the way he is because of how his dad and brother treat him.
Hero with Bad Publicity: He's the only person who can get the word out that there's still an alien spy among the school staff, but because of his reputation as a troublemaker, few people will give him the time of day. Later, when he becomes super smart, he figures out the solution to problems such as world hunger, but because he's a kid, no one will take his ideas seriously.
The Millstone: In the first book, he almost gets Peter and Susan busted by accidentally turning on Broxholm's communication device.
Reformed Bully: Starting when he discovers that Peter and Susan were right about their teacher being an alien.
Stay with the Aliens: The series ends with he and Susan joining the aliens in traveling the universe.
Would Hit a Girl: He punched Susan in the face once when she tried to break up a fight between him and Peter, although it was an accident. At least, he claimed so at the time. Susan suspects it was deliberate, his way of telling her to mind her own business.
You Are Better Than You Think You Are: The nature of Duncan's character development as the series progresses. Convinced by his family that he's little more than a brainless thug, Kreeblim explains that the brain fryer didn't give Duncan anything that didn't already have the potential to be there. Once the crisis is resolved, Duncan begins actively trying harder to live up to his potential.
Broxholm
Achilles' Heel: He can't stand Earth music, which gives Susan a way to coerce him into revealing himself at the end of the first book. Peter mentions later that he's since had special ear filters installed to prevent this sort of thing from happening again.
Big Bad: Of the first book, although it turns out he's not actually evil.
Brutal Honesty: He comes off as tactless and gruff. As it turns out, however, on his planet being brutal with honesty is normal (although Broxholm describes it as just telling the truth and then getting on with things), and in fact in comparison to the rest of Broxholm's kind, Broxholm is a "pussycat". But even the "pussycat" Broxholm cannot understand what he sees as the humans' obsession with talking nicely all the time.
Hero Antagonist: He's the main antagonist of the first book, but he has good intentions.
I Know You Know I Know: He plays this game with Susan after learning somehow that she has snuck into his house multiple times.
Jerk with a Heart of Gold: He's overly strict and grumpy, but some of that is cultural and some is him intentionally adopting a different demeanor than the kinder Kreeblim to study contrasting teaching styles. He eventually opens up to Peter about what he's doing and why, and the two of them embrace and sob over the horrible fate that may await earth.
Broxholm: Oh, you poor people. You poor, sad, wonderful people, so full of love and hate, hope and horror, sorrow and need.
Screw the Rules, I'm Doing What's Right!: Despite it being a part of his job not to directly interfere with human activity, in the fourth book, when exposed to a man about to subject a woman to tortures the narrator flatly refuses to name (implied to be rape), Broxholm snaps, knocks the man unconscious, and frees the woman.
Stern Teacher: He is very strict and boring in his "Mr. Smith" persona, especially compared to Ms. Schwartz, the teacher he replaced. He later explains that while he is naturally gruff, he deliberately turned it up a notch as part of the experiment he was involved in — to see how kids reacted to different teaching styles.
Super-Strength: Vastly stronger than a human of his size. He can pick up people like they weigh nothing, and squeeze a textbook hard enough to leave a permanent hand print.
Tyrant Takes the Helm: He replaces (actually, kidnaps) Ms. Schwartz and basically destroys everything that made her class fun.
Kreeblim
Chekhov's Gun: The skin glove she uses to hide her alien hand, which Duncan finds in the trash and uses to hide an ink stain from a fire alarm.
Rubber Man: Her arms can stretch like rubber, as Duncan notes when he sees her reaching for a bowl on a higher shelf.
Narration: When I say stretching her arm, I don't mean like you or I would stretch. Her arm actually got longer, as if it were made of rubber or something.
Hoo-Lan
Cloud Cuckoolander: Even the other aliens consider him odd. The fact that he wears Bermuda shorts is just the start.
Disguised in Drag: In book 4, he disguises himself as a girl named Sharleen to blend in on Earth.
Mentor Occupational Hazard: Subverted — he slips into what seems to be an irreversible coma as a result of coming into contact with Peter's brain, which turns out to be far vaster than he expected, but turns out to be fine in the end.
My Greatest Failure: He gave humanity the secret of television, in an attempt to stall scientific progress. It worked, but he regrets it terribly because humans weren't ready for such a powerful communication tool.
Trickster Mentor: Has a tendency to teach Peter without explaining what he's doing and why.
