Complex sentence: Another member of the staff that made "Kanon" was OdiakeS, an outsourced composer who has since helped Key with two music albums, one each released for "Air" and "Clannad", but has done nothing with Key since 2004.
Split into simpler sentences: Another member of the staff that made "Kanon" was OdiakeS, a composer who has since left Key. OdiakeS returned to help in two music albums, one each released for "Air" and "Clannad", but has done nothing with Key since 2004.