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The answer is guitar. Finneas O'Connell, also known mononymously as Finneas, is an American singer, songwriter, musician, and record producer. He is best known for his work with his sister, Billie Eilish, for whom he produces, co-writes, and plays multiple instruments, primarily the guitar. Finneas has also released his own solo music, including the EP "Blood Harmony" (2019) and the album "Optimist" (2021).