Léonin
Léonin (lived in the late 12th century) is the first composer we know about who wrote organum, a kind of church music.  We know nothing about his life.  He was probably French.  He seems to have worked at Notre Dame Cathedral because he wrote a collection of music called Magnus Liber (meaning: Big Book), or, at least, he wrote some of it.  This book has music for two voices.  It was designed for services at Notre Dame.  A younger composer called Pérotin did some more work on the collection, changing some of it.