who's polymnia?

POLYHYMNIA (or Polymnia) was one of the nine Mousai, the goddesses of music, song and dance. In Classical times--when the Mousai were assigned specific artistic and literary spheres--Polyhymnia was named Muse of religious hymns. In this guise she portrayed as a woman standing in a pensive or meditative. Her name was derived from the Greek words poly-, "many," and hymnos, "praise" or "hymn."

PARENTS
ZEUS & MNEMOSYNE (Hesiod Theogony 1, Apollodorus 1.13, Diodorus Siculus 4.7.1, Orphic Hymn 76)
ENCYCLOPEDIA

POLY′MNIA or POLYHY′MNIA (Polumnia), a daughter of Zeus, and one of the nine Muses. She presided over lyric poetry, and was believed to have invented the lyre. (Hes. Theog. 78; Schol. ad Apollon. Rhod. iii. 1.) By Oeagrus she became the mother of Orpheus. (Schol. l. c. i. 23.) In works of art she was usually represented in a pensive attitude.

Source: Dictionary of Greek and Roman Biography and Mythology.