Poor Communication Kills: Having a Bene Gesserit for a lover isn't much use when you think she's a spy. The Harkonnens expertly manipulated everybody into not talking to the Duke. He in turn is trying to draw the enemy out, but it only makes matters worse.
Real Men Hate Affection: Adding to Leto's problems, Jessica is chafing under his frosty persona. Leto desperately wants to be affectionate with her, but it would be political suicide. She's not a Duchess, just a concubine.
Taking You with Me: He attempted to kill Baron Harkonnen using a poison gas in one of his molars that would take out everyone in the room, including himself. Unfortunately he doesn't quite get the clean sweep, and that's as bad as not trying.
Trap Is the Only Option: Though outwardly a gift to House Atreides, Leto fully realizes that Arrakis is a trap. The Emperor feels threatened by Leto's popularity, and conspires to crush him.
Mama Bear: She is quite protective of Paul, and will do anything to protect him.
Mind Manipulation: Used rarely, primarily in self-defense or in a demonstration of her secret powers when such is necessary.
Minored in Asskicking: She may major in political manipulation, but she can take down any Fremen— who are the guys that easily beat up the Sardaukar. In fact she's so good that when he first meets her, Stilgar is worried that in the Fremen's Asskicking Equals Authority society, she may choose to become the leader.
Stop Worshipping Me: What he honestly starts wishing for (towards most of the Fremen) since the events portrayed in Dune Messiah. For one he weeps that the strong and independent friends that he had come to love were caught up in the fever too.
You Can't Fight Fate: And he can sense it constantly. Paul makes several attempts to avert the Jihad; thanks to his efforts, he can say with satisfaction that the fremen 'only' killed sixty billion people.
Orwellian Editor: Being a Kwisatz Haderach, has full access to his masculine and feminine genetic memory which stretches from about the year 100,000 back the beginning of human awareness. What does he do with it? He has historians burned alive for misconstruing the facts that he has personal access to. Of course, this was partly mystique-building, as he secretly rendered them unconscious first.
Silent Scapegoat: Even the Bene Gesserit, thousands of years after Leto sacrifices himself, don't realize just what it was he was trying to accomplish.
Sorcerer King: See his father's entry. How "good" they are is debatable, considering how many billions died during the jihads of Paul's Fremen and how Leto played tyrant in order to get people to leave the stagnant core worlds and scatter throughout the universe. They have the ability of prescience and conscious control over their bodies to an improbable level. And of course Leto II merged with a sandworm and became the Trope Namer for God Emperor.
The Mole: The in-universe equivalent to Judas Iscariot. He doesn't relish the prospect of being remembered as a notorious traitor.
The Power of Love: A very dark example. In the Duniverse, doctors of the Suk School undergo mental conditioning so they really can do no harm, but Yueh's feelings for his wife were so strong it allowed him to overcome it. It's left open whether this is really due to love or the fact that his wife was a Bene Gesserit and the whammy that they habitually put on everybody the come in contact with was just stronger than the one he got from the Suk School.
Taking You with Me: Yueh is unsure of whether his wife is alive, but plans to take out the Baron in either case.
"Ahhhh, Gurney! He could kill you while singing and never miss a note."
Another Atreides retainer, he serves as the House's swordmaster, responsible for managing the household's defenses. He dies in the first book, only to be reincarnated over and over again in later books.
Ascended Extra: He appears more than any of the main characters of the entire series.
Fremen Naib (chief) who mentored Paul in the ways of the Fremen. After Paul's ascension to Emperor, he becomes one of Paul's generals and the guardian of his children. At the end of Children of Dune, Leto II bestows upon him the title of "Naib of Naibs".
Depraved Homosexual: He has slave boys delivered to his quarters, and the Dune Encyclopedia, though not canonical, hints that his attractions may extend to Feyd-Rautha. There are hints of it in the David Lynch film as well. He certainly had designs on Paul, but gave them up when he realized how lethally dangerous the boy was.
Even Evil Has Standards: Would have preferred to torture Leto in private, one nobleman to another. For one, it sets a bad precedent for the next ruler of Arrakis. Also, if word got back to the other Houses that the Imperium is involved, everyone would be terrified the Emperor's coming for them next. However, the Baron quickly loses his patience when Leto doesn't bend.
Luke, I Am Your Father: One of his cast-off concubines (there are hints it's Reverend Mother Mohiam; this is hinted at in the David Lynch film and confirmed in the prequels) gave birth to Jessica, making him Paul's grandfather.
The Social Darwinist: His soliloquy about rabbits, and how fear and power are the tools of statecraft, mark him as this.
Villainous Incest: He actually ogles his nephew, Feyd. To say nothing of his attraction to Paul.
We Have Reserves: His reaction to Leto's final attack is to close the door on his own men, leaving them to die from poison gas. (In the miniseries, he simply flies away from the range of the gas.) His immediate reaction was joy that they had died and he survived. He also casually kills off Leto's troops rather than recruit them, but then again, they were hardly likely to serve him.
Piter de Vries
Ambiguously Gay: Very effete and Baron Harkonnen repeatedly insinuates that he's not really attracted to Jessica and only desires her as a status symbol.
I Have You Now, My Pretty: He intended to do this, but the Baron convinced him that raping Jessica was a really stupid idea (more exactly, he was offered Jessica and exile— in case she turned or killed him— or Arrakis, and his love of power won).
Badass Normal: Holds his own against the Kwisatz Haderach. Questionable how normal he is though. He's had the same intense level of combat training that Paul has, with added lessons in how to fight dirty in a nominally "clean" contest.
Bastard Understudy: The Baron is grooming him to be the next ruler of Arrakis, and perhaps even Emperor.
Count Fenring laments that Feyd was raised the way he was; if he hadn't, Feyd could have turned out like Paul. He is of course, Paul's first cousin
Feyd is also the remaining male from the Bene Gesserit genetic selection line (discounting the renegade Paul), making him their last hope (except for Feyd's bastard daughter). Furthermore, he was most likely supposed to be one of the parents of the Kwisatz Haderach, with Jessica and Leto's child being the other.
Let's Dance: The duel opens with Feyd complimenting Paul's "dancing."
Not Cheating Unless You Get Caught: "On his seventeenth birthday, Feyd-Rautha Harkonnen killed his one hundredth slave-gladiator." That's because each of those slaves was drugged or programmed to respond to Safe Words. And of course, his weapons are all poisoned. This time he faces one of Leto's elite troopers, and he very nearly winds up dead despite the deck being stacked in his favour.
Smart People Play Chess: The Baron gingerly broaches the topic of the latest attempt on his life (by Feyd) by pointing out the Pyramid Chess set in his room.
The Starscream: The Baron is aware of Feyd's continual attempts to kill him, but can do nothing.
Know When to Fold 'Em: In the miniseries, when he was attacking the Harkonnen city (presumably on the Baron's orders), he is encountered by the citizens who were present. When he realizes that he's not only outnumbered, but he's going to suffer immediate death as a result and not capture, his reaction is to drop his weapon and scream to the heavens, not doing even one thing to delay his death. His death is not described in the books.
Off with His Head!: His ultimate fate in the miniseries, and in the David Lynch film (albeit at a different set of hands).
The Baron is convinced that he's a dolt. However, Rabban was regent of Arrakis for years and learned a thing or two — namely, don't mess with the Fremen.
In the first novel, he suggested to the Baron that they perform a count of the Fremen on Arrakis, which the Baron immediately rejects. If the Baron had listened to his nephew, he might have been better prepared to fight the Fremen later.
And Rabban does get in a very astute question to the Baron regarding the Baron's subversion of Yueh, which causes the Baron to at least wonder how smart Rabban really is.
The Exile: After abdicating the throne to Paul, Shaddam is banished to the Sardaukar prison planet.
Friendly Enemy: In secret, Shaddam admires his cousin Leto, and had once mentioned to Irulan that he had wished the political climate that necessitated wiping out the Atreides did not exist. As it stands, the Emperor orders Leto's death simply for having a more powerful army than him.
Laser-Guided Karma: The Emperor boots Leto out of Caladan, only for his heir to return and unseat him from the throne.
Properly Paranoid: He suspects the Harkonnens of moving against him... which is actually the case. In fact, the Baron anticipated Corrino's wrath and planned to unify the Houses against him.
The Social Darwinist: The Emperor is known for his open contempt of "weakness". When the Sardaukar find Leto's body, the Baron flies into a panic — not because he killed Leto, but because he botched the job. Word of the poison gas will get back to the Emperor, who will make life difficult for him.
Count Hasimir Fenring
A counselor and friend of Emperor Shaddam; also an assassin. Was one of the Bene Gesserit's recent failures.
Badass: Implied. After the Emperor sees Paul fight he concludes that Fenring would be able to take him easily even if he wasn't exhausted.
Stockholm Syndrome: Shows a remarkable amount of devotion to Paul considering how he treated her. Not abuse exactly, but "not one whiff of tenderness" is a bit hard. (Then again, she knew the score. If she'd been significantly older, her father would have married her to Leto Atreides.)
The private army of the Padishah Emperor.
Always Someone Better: The Sardaukar are so good because they are trained on the Death World of Salusa Secundus. Leto (and Paul) correctly figure out that Arrakis is an even worse Death World and so its inhabitants, the Fremen, will be able to beat the Sardaukar.
Paper-Thin Disguise: Shaddam Corrino IV sends two full legions of Sardaukar in the guise of Harkonnen soldiers to bolster their assault on the Atreides after Yueh's bertayal. The Atreides pick up on this almost immediately, recognizing the incredibly distinct and vicious fighting style of the Sardaukar. The Sardaukar even attempt disguising themselves as Atreides soldiers during the assault.
Weaponized Exhaust: The Sardaukar used this to barely eke out a victory/capture of some Fremen.
We Have Become Complacent: For many years, the Sardukar were able to coast on their reputation as being unbeatable in combat, and thus their skills atrophied, though they were still a force to be reckoned with by the time of the first book.
Worthy Opponent: To the Fremen, at least in comparison to Harkonnen soldiers.
Reverend Mother Gaius Helen Mohiam
Compelling Voice: Like all Bene Gesserit, Mohiam is trained in the use of the Voice.
The leader of the Tleilaxu in Heretics of Dune, he seeks Tleilaxu hegemony and attempts to make allegiances with the Honored Matres and Bene Gesserit in pursuit of his goal.
Out-Gambitted: Every attempt by Waff to use the Bene Gesserit to his ends gets foiled, forcing him to join his cause to theirs in the end.
Introduced in Heretics of Dune, the Honored Matres are a militant group of women descended from the Fish Speakers, Leto II's elite cadre of all-female warriors. Using sexual and fighting techniques derived from the Bene Gesserit, they intend to unite humanity under their rule— even if they must smash a few heads in along the way.
The Thinking Machines were artificial intelligence that once enslave mankind, but they were overthrown by the humans in the Butlerian Jihad after which ai's have been banned. But the Thinking Machines were merely defeated and they are planning to rise again.