Vermouth is a member of the Black Organization. But, although she follows orders from the boss and Gin when he is in charge of missions, she often acts according to her own agenda and whims, even when it runs contrary to the Black Organization's best interests, being secretive and manipulative, even towards other Organization members. Vermouth is confident in her abilities, is very bold, and an expert at manipulating situations to her advantage: traits which allow her to single-handedly weave complex plots and take the upper hand even while she is being monitored by enemies. She is intelligent and insightful, but her personality is contradictory at times. On one hand, she is a cruel and ruthless agent who will blackmail an unwilling man to commit murder by threatening him and his family just to provide a distraction and happily tell an FBI agent she should smile because she will see her dead father again in heaven. On the other hand, she risks her life several times to protect Conan, Ran, and Kogoro, and her friendly behavior while in disguise as Tomoaki Araide left the real Araide wondering if Vermouth is really a bad person.

In the past, under the name Sharon Vineyard, Vermouth was friends with Yukiko Kudo back when they were acting together, but was even then secretly a member of the Black Organization. It is unknown for exactly how long she has been a member, but it is known that she had been secretly living at least two lives through the art of disguise: that of herself and her supposed daughter, Chris Vineyard. After staging Sharon Vineyard's death, she now lives as Chris and continues to serve the Black Organization. At one point, some twenty years ago, she killed the parents of a little girl who would become the FBI agent Jodie Starling, one of her most bitter enemies.

Vermouth is the boss's favorite which grants her some independence. And also appears not to age or have de-aged over a period of twenty years between when she killed Jodie's parents and her current appearance.

She has great personal interest in Conan, constantly calling him "Silver Bullet." Since it has been revealed that she knows about Ai and Kudo's reversion to their child forms, she seems to know the shrinking effect the drug can have, but for reasons yet unrevealed keeps this knowledge secret from the rest of the Organization.
(Source: Detective Conan World)