Abbadona

Abbadona:  A fallen angel, a seraph, at one time the chosen companion of the faithful Abdiel (q.v.).  In Klopstock, The Messiah, Canto 21, Abbadona, not wholly committed to the rebellion and constantly bemoaning his apostasy, is called "the penitent angel."  It should be pointed out, however, that a fallen angel cannot repent - not, at least, in Catholic doctrine - for once an angel sins, he is "fixed eternally in evil" and his mind, accordingly, can think evil only.  (a)

Resource List - all entries are taken verbatim from the original source:

(a) "The Dictionary of Angels" by Gustav Davidson, © 1967