|“||You and I are so alike—young, untested amongst these ... wolves||”|
–A Court of Mist and Fury, Ianthe to Feyre
Ianthe was a High Fae noble born in the Spring Court and was a childhood friend of Tamlin's. Her father was one of Tamlin's strongest allies at Court and captain of his forces. Around 300 years ago she become the youngest of the twelve High Priestess. Her father had sensed the trouble coming and packed up Ianthe and her two younger sisters and fled to Vallahan and lived there for fifty years while their people were butchered and enslaved.
Ianthe has returned from Vallahan and has taken a role in Tamlin's Court, where she acts as a companion for Feyre. She helped plan the wedding and took the role of officiating it.
Tamlin gives her the temple complex on his lands to occupy and renovate.
Ianthe convinces Tamlin not to allow Feyre to learn how to fight. She tells Feyre that "every word from her mouth, every turn of phrase, will be judged, and possibly used against her."
When Feyre returns to the Spring Court, Ianthe apologizes to her, claiming that giving Nesta and Elain to the Cauldron was only to spare Feyre the sorrow of seeing her sisters grow old—and that she had to help keep the alliance with Hybern. During the Summer Solstice, she performs a ceremony but is upstaged by Feyre, who uses her Day Court magic to glow in front of the crowd. This unsettles Ianthe, who scrambles back for control and love from the Court.
A set of keys to Rosehall is later found missing. It is revealed through Feyre‘s magic that the guard on duty was lulled to sleep by Ianthe, who stole the keys and then unleashed a bunch of nagas on the manor to gain attention after the Solstice incident. Tamlin struggles to choose between the guard or Ianthe, one having served him and been willing to die for him, and the latter being a powerful ally to the High Priestesses. He chooses Ianthe, just as Feyre had planned, and whips the innocent guard. This is what ultimately makes Tamlin’s guards leave his service, and splits the Spring Court in half. Later, when Feyre is about to escape the Court, Ianthe shackles Lucien with controlling magic and assaults him. Feyre overhears their conversation and comes to Lucien’s aide. Feyre uses her Daemati abilities and forces Ianthe to smash her hand with a rock until it breaks and is cursed to never touch another person against their will again.
Later, her mind is healed by the King of Hybern, but she still can’t use her hand. Ianthe kills the Suriel and tries to kill Feyre, but Feyre manages to trap her in the Weaver’s Cottage. It is assumed that the Weaver killed Ianthe because when Feyre meets her in battle, the Weaver is wearing Ianthe’s jeweled band.
Ianthe has long, gently curling, golden hair and a slim waist, usually hidden under her pale blue-gray robe. On her forehead, above her brows and teal eyes, she has a tattoo in midnight-blue ink of the different stages of the moon cycle. She has a set of silver rings that she wears on her manicured fingers. Her typical wardrobe consists of a robe with a fine belt of sky-blue, limpid stones, each perfectly oval and held in shining silver. On top of her hood, she wears a matching delicate band of silver. In the middle of the silver band is a large blue stone. A panel of cloth is folded up beneath the circlet. It's a built-in swath meant to be pulled over the brow and eyes when praying. Ianthe has a full and sensuous mouth and a pert nose. She is described as very beautiful. Her voice is sweet and feminine. As a High Fae, she has elegant features and a supple body. She wears a set of silver bracelets that tinkle together.
Ianthe is smart, cunning, and ambitious towards her goal of ensnaring a wealthy courtier or High Lord for marriage and offspring, whether they want her or not. She knows exactly what she wants and will go to great lengths to get it, including manipulating others and forcing people to do what she wants. She was a benefit to Tamlin in the Spring Court as she was aware of everyone in Court and outside of it. She hurt Feyre by always siding with Tamlin and being completely and utterly supportive of the wedding, but manipulated her into believing she was her close friend.
Feyre befriends Ianthe after returning from Under the Mountain. She worships her and Tamlin as if they were "newly minted gods, blessed and chosen by the Cauldron itself." She stays with Feyre at events and dinners to tell her about those in attendance. Feyre is more than happy to hand over the power to make decisions regarding events in the Spring Court, including Winter Solstice and Feyre and Tamlin's wedding. Feyre even lets Ianthe pick out what clothes she wears. Feyre admits that she has let Ianthe become a crutch, but Ianthe seemed willing to be one, caring when she couldn't. Feyre hates that Tamlin sides with her, even when he agrees with Feyre.
After leaving the Spring Court, and befriending Rhysand's Inner Circle, especially Mor, Feyre realizes that Ianthe was never truly her friend. She despises her for how she treated Lucien and Rhysand.
The first interaction we see between Ianthe and the High Lord of Night is at Feyre’s wedding. When Rhysand arrives to disrupt the wedding, Ianthe backs away from the alter, her face drained of color.
Later, Rhysand shows Feyre a memory of Ianthe from around one hundred years prior to the events of A Court of Thorns and Roses, in which Ianthe tries to persuade Rhysand into an alliance. She first flirts with Azriel and many of the other males in his Court before going after Rhysand. Rhysand discovers Ianthe lying naked in his bed. Ianthe tries to get Rhysand to sleep with her. Her true motives are revealed when she tells Rhysand their offspring could rule Prythian. He continuously refuses her advances until she tries to touch him inappropriately. He then breaks her hand with his magic and tells her to never touch anyone in his Court again.
Ianthe shows a continuous interest in Lucien during her stay in the Spring Court, even though he doesn't reciprocate. She often finds ways to bring him up in conversation, to corner him at events, and to touch him. Her interest in him is due to his powerful lineage and the prospect of conceiving his children. While Feyre was living in the Night Court, Tamlin refused to take part in Calanmai so instead Lucien took his place, and at Ianthe's insistence, completed the Great Rite with her. Ianthe blurs the line between willingness and duty, giving her some power over Lucien and making her feel entitled to him.
While on one of the scouting trips with Prince Dagdan and Princess Brannagh, Ianthe tricks Lucien and shackles him to a tree to force him to accept her sexual advances. Feyre interrupts them and Lucien is released, witnesses Feyre's attack on Ianthe, then escapes with her.