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", an 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.
Cast as a Mask: Inverted and has some gender bending thrown in in Revelation.
Complete Monster: While his brother eventually feels remorse for his actions and does a Heel Face Turn in Revelation, Sirrus not only seems to have no regrets but commits additional atrocities: kidnapping his sister, taunting her about how he plans to suck her soul out of her body, his plans to murder both her and their parents, and in the bad endings he murders his brother and the player.
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.
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.
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.
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.