Parsifal (also known as Perceval) was a knight of the Round Table in the Arthurian legends. Because of his chivalry and purity, Parsifal was able to get a glimpse of the Holy Grail.
Jessie L. Weston's book on the Grail legend From Ritual to Romance was (mentioned in Eliot's first note to The Waste Land) has much on Parsifal, the Fisher King and the myth of the Holy Grail. Weston notes the change in hero of the Grail myth. In the older myths the hero was Sir Gawain but he was later replaced by Sir Parsifal who in turn was replaced by Sir Galahad as the hero of the Grail quest in the Arthurian literature starting in the 13th century.
The text of From Ritual to Romance is available online at the Celtic Twilight web site for Arthurian legends at http://www.celtic-twilight.com/
Richard Wagner wrote the opera Parsifal in 1882.
Note that there are two references to another Wagnerian opera (Tristan und Isolde) in The Waste Land
at lines 31 - 34
31) Frish weht der Wind
32) Der Heimat zu
33) Mein Irisch Kind,
34) Wo weilest du?
and line 42
42) Oed' und leer das Meer.