[Fingolfin II]

Right, I'll go for the less obvious one- was it Glorfindel?

Balrog's bane- killed the Balrog he fought with on the mountain peak.
Came back again- was reborn and a part of Elrond's household (though this isn't certain, that's true).
Twice aided a flight, from the forces of night- he helped Frodo escape the Nazgul, and aided the escapees of Gondolin.
Person of might- described as an Elf-lord of the Firstborn, who has dwelt in the Blessed Realm.
Clad in white light- was shown to be 'an Elf-lord revealed in his wrath', when the Nazgul tried to cross the Bruinen, as he was a shining white figure.

I doubt this is the right answer, but I must cover all contingencies!