Mrs. Coulter is 35 years old in the third novel,
The Amber Spyglass. She is described as being "beautiful and young" with "sleek black hair" that "framed her cheeks" and slim, though later accounts of her describe her as having blonde or "golden" hair. However, by the third book she finds herself torn, ultimately feigning loyalty to betray the Church and finally giving up her life to save Lyra.
Metatron, the Regent of the Authority describes her character in "the most searching examination Marisa Coulter had ever undergone", stripping away "all shelter and deceit" and describing that he sees: :
"Corruption and envy and lust for power. Cruelty and coldness. A vicious probing curiosity. Pure, poisonous, toxic malice. You have never from your earliest years shown a shred of compassion or sympathy or kindness without calculating how it would return to your advantage. You have tortured and killed without regret or hesitation; you have betrayed and intrigued and gloried in your treachery. You are a cess-pit of moral filth". Despite this, Metatron describes her as lovelier than any wife he had, and he wishes to take her as his
consort. : ''"I told him I was going to betray you, and betray Lyra, and he believed me because I was corrupt and full of wickedness. I wanted him to find no good in me and he didn't. There is none. But I love Lyra. Where did this love come from? I don't know; it came to me like a
thief in the night, and now I love her so much my heart is bursting with it. All I could hope was that my crimes were so monstrous that the love was
no bigger than a mustard seed in the shadow of them, and I wished I'd committed even greater ones to hide it more deeply still"''. Mrs. Coulter's dæmon takes the form of a golden monkey with long fur, who is not named in the books, but was given the name "Ozymandias" in the radio adaptation. The golden monkey is shown to be capable of going much further from Mrs. Coulter than other dæmons are able to
separate from their humans. How the golden monkey can go so far from Mrs. Coulter is not explained, Mrs. Coulter has not undergone any of the processes which enable other humans to achieve such separation. Her final act in the trilogy occurs when she is reconciled with Asriel, and they together drag Metatron into an endless
abyss, the three thus ceasing to exist. ==Relationship with Lyra Belacqua ==