The Siren, a quite powerful GF can be drawn (using GF draw ability from GF junction) from Elvoret during the fight with it on the communication tower (disc I) before escaping from indestrustable X-ATM092.

If you have forgotten to draw it from Elvoret, you can either draw it from from Tri-Point in the Wind Cellar of the Ultimecia's Castle (disc IV).

It's special power is "Silent Sound" that causes a little non-elemental damage against enemies (rarely on bosses) plus 'silence'.