Game Over Man: If you foolishly enter D'ni without the green page, leaving you both without any hope of exit. This actually causes Atrus to shout "fool" at you before turning away in disgust. (If you instead choose to free Sirrus and Achenar, they will gloat before trapping you in their respective books.)
Properly Paranoid: At the beginning of Exile, Catherine complains about Atrus' obsession with security. As it shortly turns out, his efforts are both entirely justified and woefully inadequate. (He thinks someone's been breaking in, which is true, but what he doesn't know is that it's not someone in Tomahna, it's someone in a completely different Age who happens to have a Linking Book to Tomahna that gets them right past his locks.)
Set Right What Once Went Wrong: Non-time-travel variant; Gehn isn't a particularly good linking author, so the quantum-uncertainty thing the linking books have going on makes the Ages he links to dangerously unstable. Atrus is much better at writing linking books than his father, and reckons he can use those same quantum-uncertainty shenanigans to salvage Gehn's Ages, or even undo the damage Gehn caused; but it's a very time-sensitive endeavor because Ages don't stop deteriorating just because you're not in them, and some are beyond saving already. In fact, this is how Atrus contributes to your efforts to save Catherine and capture Gehn: The entire time you're in Riven, he's writing frenziedly in the book to patch any errors he can find and make sure that the Age doesn't fall apart and kill you.
What the Hell, Player?: Atrus doesn't react well to your stupidity if you go to D'ni without bringing the missing linking book page.
Nor does he react well to your cruelty if you trap Saavedro in Narayan and leave without letting him go.
Atrus' wife and a native of the Riven age.
Beyond the Impossible: In the novels she's mentioned as having written "Torus", a stable doughnut-shaped Age, which features a huge waterfall that falls through the planet's core, turns into rain and gets carried back by clouds to refill the ocean, that in turn feeds the waterfall. Atrus' reaction to first seeing this was that, until then, he had thought it impossible to do such things with the Art.
Killed Off for Real: His body is an Easter Egg you can find if you look at the metal tube once you enter his hideout. You see Sirrus sleeping in the tube. Even then, Sirrus isn't dead - only his mind is, as solving the memory lock he holds on Yeesha essentially destroys his soul, rendering Sirrus a vegetable forever.
Obviously Evil: From the moment he makes contact with you, it's blatantly obvious he's unhinged and unstable. His rooms in the various Ages are also filled with torture devices, and he has a closet filled with the mutilated bodies of his victims. This is in contrast to Sirrus, who behaves calmly and rationally and whose rooms show him to enjoy wealth, which doesn't seem so bad until you realize that his opulence stands in stark contrast to the sparse dwellings of the natives around him, and that wealth had to come from somewhere...
Villainous Breakdown: Suffered one in Haven. After realising he was trapped on his own with no Linking Book, his already unstable mind underwent a complete psychotic meltdown, causing him to butcher his way through half the Age's animal inhabitants in blind rage. Ultimately subverted, as he eventually came to terms with his fate and realised with all the blood on his hands just why he deserved to be there. Eventually, this led to his redemption.
Atrus' father and the main antagonist of Riven
A God Am I: Has a massive god complex. Any number of the D'ni had this problem as well.
Even Evil Has Loved Ones: Most of his diary is filled with pompous declarations and ranting... until you reach the page where he grieves for his wife whom he has not seen in years. There is a teardrop strain in the corner.
Faux Affably Evil: He is softspoken and unfailingly cordial in your conversations with him, and he claims that he's a better man for his time trapped in Riven - even expressing regret for trying to kill Atrus during the backstory. It's clearly just a mask to disguise his true ambitions: if allowed, he'll gladly shoot Atrus dead, shoot you as well, or even leave you in the Trap Book for all eternity. During a bad ending he goes so far as to politely apologize for shooting you with a poison dart and muse nostalgically on the chance of seeing Atrus again while you slowly die in the background.
Game Over Man: Takes over for Atrus in this regard. If you release him from the trap book, he will puzzle over your naivete and force you to take his place. Likewise, if you set him loose on Riven, he orders his guards to plug both you and Atrus with poison blowdarts.
Genre Savvy: When you finally meet him, Gehn doesn't try to refute anything Atrus told you about him, doesn't try to bribe you, and doesn't try to glorify his evil deeds. He instead says he's grown remorseful over his past actions during his long imprisonment, and is currently trying to protect the people of Riven from the Age's impending collapse and a small group of terrorists. Of course, he's blatantly lying, but he manages it more convincingly than most, especially considering the strong emphasis on redemption seen in the series which might lead a Wrong Genre Savvy player to take him at his word.
Fate Worse than Death: In one of his rants to Atrus, he mentions how Sirrus and Achenar destroyed his civilization and separated him from his family, concluding that "It would have been better if I had died." In one of the endings, you can put him in an even worse fate than the one Sirrus and Achenar put him through.
I Have Releeshahn: Holds the only contact Atrus has to his people hostage in order to get him to fix what Sirrus and Achenar did 20 years prior.
I Lied: Saavedro offers to return Releeshahn to the player freely at the end of Myst III when it turns out he can't return home without help. If you take up his offer directly, he gleefully tosses the book into an abyss and scampers off home.
Creepy Child: Later part of Myst IV - Sirrus possesses Yeesha's body. He does a good job at impersonating her, but his language slips through. Comes to a head in the bad endings, in which she smiles sweetly as she shoots you with a crossbow.
Daddy's Girl: Her father always calls her "my desert bird". In End Of Ages, when everything falls apart, they still have each other.