Sirrus (D'ni: Sihrus/[Sirus]) is the youngest son of Atrus and Catherine.

Sirrus and Achenar were two bright young boys who grew to be confident, ambitious young men. They followed their father Atrus everywhere and had many journeys together. The two boys were very intelligent, and created many useful devices and projects to help their father in the ages he linked to. As time passed though, Atrus gave them free reign to the books all describing and linking to the fantastic ages that he had visited. However, Atrus later remarked that this was probably unwise. Unsupervised as they were, they began to change. One or both of them ended up destroying and burning all but four of the ages of Myst, their home island which was the gateway to many others.

Sirrus was the brother who was lustful mainly for beauty, power, wealth and luxury. As with many people of this type, he had also an addiction to alcohol and drugs.

In Myst, Sirrus was first met INSIDE the red book on Myst Island in Atrus's library. The book was revealed to be a type of trap or prison that Sirrus claimed that his father had encarcerated in. At first, the book has many pages missing, and when first opened without the pages, the player can not see or hear Sirrus at all due to the static blocking the gateway image. When the first red page is put into the book, Sirrus is able to barely see someone looking into his red book, he implored the player to find more red pages which were scattered in the four remaining ages of Myst. With each page he could see out of the book more clearly. As the vision became clearer he shouted out not to pick up the blue pages belonging to his brother Achenar's blue book. He slowly explained through the static which later cleared once enough pages were gathered, that he could could escape and reward the player. [1]

At the end of the game, the player frees Sirrus and Achenar's father, Atrus, from his prison in D'ni. The player waits in D'ni, and when they return to the island of Myst, the red and blue books that acted as the two son's prisons have mysteriously disappeared, leaving burn marks in their place.

Later, in Myst IV: Revelation, it is discovered that the brothers are in fact not dead. With the release of this installment of the Myst franchise, curiousities and plot holes involving the brothers shall be cleared up.