Because You Were Nice to Me: She quickly develops a sisterly affection for Harvey, because he is much nicer and more welcoming to her than Lloyd when they first meet.
Establishing Character Moment: When she first meets up with Lloyd and Harvey, she refuses the crisps and coke they offer her, then rather shyly tells them her name, along with her history.
Happily Adopted: While there was some initial tension with Lloyd in particular, Dinah soon became close to him and Harvey, and Lloyd in particular spoke up in favour of Dinah being officially adopted. Dinah later affirmed that she can't imagine anyone being more of a mother to her than Mrs Hunter, and even when she believed she had a chance to reunite with her biological father, she made it clear that she still wanted to call Mr Hunter "Dad".
Humble Hero: Dinah is relatively hesitant about showing off her intelligence at first, such as planning out where to make her mistakes in her initial test for the Headmaster.
Meaningful Rename: She changes her last name after she is adopted by the Hunters.
Nice Girl: While Dinah can be rather shy, for the most part, she is a kind, caring, compassionate and humble person.
Teen Genius: Dinah is often shown to have a wide range of knowledge and skills. While the only member of SPLAT vulnerable to the Headmaster's powers, she makes up for this weakness by generally being smart enough to deduce his latest scheme and work out how to counter it.
Thou Shalt Not Kill: When Dinah is in a position to crash a helicopter with the Headmaster in it, she refuses because she can't bring herself to kill.
Defrosting Ice King: Initially rather harsh towards Dinah before he grows to accept her.
Hot-Blooded: Lloyd's main weakness as a leader is that he tends to get very hot-tempered, so he'll focus on trying to act rather than work out what exactly the Headmaster is trying to do.
Jerk with a Heart of Gold: He can have control issues and a short temper; but he is generally a good person, and determined to stop the Headmaster once he learns he's involved in events.
Plot Allergy: He falls into a coma in the fourth book after being attacked by a gigantic wasp.
Ungrateful Bastard: Lloyd comes off as a bit unfair on Dinah after the snowball incident considering that Dinah had responded to it by threatening the Headmaster. He realizes his mistake after learning that Dinah had been hypnotized by The Headmaster.
Harvey Hunter
Deadpan Snarker: Less so than Lloyd, but he has his moments.
Nice Guy: He is more laid-back and amiable than his brother Lloyd, which earns him Dinah's sympathy very early on.
Adults Are Useless: In the first book, she was often dismissive of what Lloyd and Harvey told her about what was happening at the school. It's somewhat justified because The Headmaster went out of his way to ensure that anybody who could back up Lloyd and Harvey's claims wouldn't be able to by hypnotizing them.
Good Parents: While she can't be much help directly to her children against the Headmaster, she is a kind, loving mother towards all of them.
Heel–Face Brainwashing: Inverted. She ends up getting brainwashed into going back to her old ways when The Headmaster sends a hypnotic message to the parents of Hazelbrook's students.
High-Heel–Face Turn: In the fourth book, when she finds herself breaking his control.
Jerk with a Heart of Gold: By the fourth book. During the fire in the tunnel at the climax, she prays that the members of SPLAT will get out safely and is horrified along with the rest of SPLAT and Simon when Eve dies prematurely.
Even Evil Has Standards: Despite being under the Headmaster's control, she almost quits his service when Hyberbrain goes out of control.
Heel–Face Turn: The moment she sees the lobotomized Mrs. Hunter, she breaks free of The Headmaster's brainwashing.
The Headmaster
Ambiguously Human: He has strange hypnotic abilities and a fixation with enforcing order on the world, but he is never explicitly implied to be anything other than human.
The Comically Serious: Exploited at the end of the first book; SPLAT defeats him by making him look ridiculous right at the crucial moment.
Catchphrase: "Funny you should be so tired, so early in the morning."
Child Hater: Has a very low opinion of children, considering activities like playing to be a waste of time.
Control Freak: His primary goal is to impose his view of order on the world, to the point where SPLAT have described him as wanting to make the world a machine.
Covered in Gunge: His plan in the tv version of the first book is ruined when Dinah covers him in goo. (pepper in the book)
Deadpan Snarker: For someone who despises all forms of frivolity, he occasionally shows a very dry sense of humor.
Didn't Think This Through: Occasionally, such as in "The Prime Minister's Brain" where he programmed the Super Salamada to keep out anyone with the surname "Glass"; he didn't anticipate that Dinah had been renamed Dinah Hunter after the Hunters adopted her, so ended up getting in anyway.
Evil Cannot Comprehend Good: His complete rejection of emotion means that he cannot understand human gestures based on kindness or fun, allowing SPLAT to take him by surprise on occasion because he can't anticipate how they will react.
Evil Principal: It quickly turns out he can hypnotize anyone who looks at him, causing the main character to join a group of students dedicated to opposing him. The later books avert this, by having him engage in wilder supervillain plots, like attempting to gain access to the Prime Minister, and cloning people (albeit, not dropping the title).
Four Eyes, Zero Soul: Of course, what's behind those glasses is a whole lot scarier.
Hypnotic Eyes: The Headmaster can essentially control anyone so long as he can look them in the eyes, barring the few exceptions such as the original five members of SPLAT.
Knight Templar: He really does think the world would be better off if everyone were like him.
Laser-Guided Karma: He gets struck with this at the end of every story, his plan thwarted and forced to retreat.
Lean and Mean: He is generally presented as tall and thin, but undeniably ruthless.
Mass Hypnosis: Attempts this at the end of the first book, and uses technology to put it into practice in the third and fifth books.
No Name Given: The Headmaster is only ever referred to by his title (although he was referred to as 'the Director' in the fourth book in his role as controller of the BRC), but the second and fifth books in particular make it clear that he does have a name and that SPLAT know what it is.
Not So Above It All: His scheme in Revenge Of The Demon Headmaster involved him becoming a Kids' Show Mascot Parody. This clashes somewhat with his serious and anti-fun demeanour, but he explains that his plan is to inspire an obsession and then instigate riots by ensuring that there is no new merchandise available.
Pepper Sneeze: His plan in the first book is thwarted when Dinah covers him in pepper, making him sneeze.
Pragmatic Villain: By his usual standards, his plan in Revenge of the Demon Headmaster went against his usual obsession with order as he deliberately planned to instigate riots and chaos. When Ian challenged him about the apparent contradiction, he clarified that he only dislikes such actions if they are "meaningless", and these planned riots would have a purpose of allowing him to step in and restore order by providing what people seek.
The Spook: It is never revealed where the Headmaster acquired his powers or his fixation with order.
Straight Edge Evil: He doesn't appear to have any vices or self-indulgences besides the whole "obsessed with world domination" thing.
Would Hurt a Child: He has often shown a willingness to hurt the members of SPLAT, ranging from ordering his other "servants" to tear them apart as old dolls to having them wrapped up and shipped away as carpets.
The Giant Wasp
Wicked Wasps: Is very hostile to people who go near the research centre at night as seen when it nearly stung Lloyd to death.
Phil Ashby
Identity Amnesia: He lost his memory of his real name over 10 years prior to the book.
Luke, I Am Your Father: Subverted. Dinah ends up believing that Phil's her long lost dad due to the fact that the Headmaster used a picture of her father to trick her.
