Raffaela "Raffi" Musiker was a female Human who lived in the late 24th century. She once served in Starfleet under Admiral Jean-Luc Picard, and years later joined him on an unsanctioned mission to seek out Soji Asha and Bruce Maddox.
As a lieutenant commander, Musiker helped Picard formulate a new plan to evacuate Romulans from the Romulan supernova after the 2385 Attack on Mars destroyed the Romulan rescue armada. She gathered information for a proposal to use mothballed ships and reserve duty personnel to carry out the operation instead. However, Starfleet rejected the proposal and accepted Picard's protest resignation. Musiker was distraught, as she believed she would be dismissed from Starfleet as well. She also suspected Tal Shiar involvement in these events, but admitted that she could not discern a plausible motive. (PIC: "The End is the Beginning")
La Sirena and Picard's mission
After Picard's encounter with Dahj Asha in 2399, Picard reached out to Musiker to enlist her support for a mission to seek out Bruce Maddox. She was reluctant to speak to Picard until he revealed that Romulan assassins were operating secretly on Earth. (PIC: "Maps and Legends", "The End is the Beginning")
She supported his search, hiring old contact Cristóbal Rios and his ship La Sirena, and coming along to Vashti, Freecloud, Nepenthe, the Artifact, and Coppelius. (PIC: "The End is the Beginning", "Absolute Candor", "Stardust City Rag", "The Impossible Box", "Nepenthe", and more)
Musiker was fond of Saurian brandy and Chateau Picard, in particular the '86 vintage. She had a habit of inhaling snakeleaf, which caused paranoia as a side effect. (PIC: "Maps and Legends", "The End is the Beginning")
Musiker considered Captain Emily Bosch "Emmy" an old friend, claiming to know where her "bodies were buried," and would call her up on occasion when she needed a favor. Musiker called on Bosch in 2399 to get diplomatic credentials for Jean Luc Picard, prompting Bosch to ask Musiker never to call her again. (PIC: "The Impossible Box")
Raffi Musiker was played by Michelle Hurd.
The character's full first name was not revealed until "Stardust City Rag", the fifth episode of Star Trek: Picard.
According to the PIC Season 1 press kit, Musiker was a veteran Starfleet Intelligence agent with experience investigating and analyzing Romulan affairs and a "formidable tactician". It also claims that after Picard's retirement from Starfleet, she returned to Starfleet Intelligence before eventually leaving the service. 
Before her first on-screen appearance, Raffi Musiker was featured in the Star Trek: Picard - Countdown prequel comic book. According to the comic, she was Picard's first officer on the USS Verity (β) in 2385, during the mission to relocate Romulans threatened by the supernova, and was noted as "Starfleet's foremost analyst on Romulan affairs."
"Musiker" is German for "musician".