Ya’akov wrestles with an angel in our parsha, and before the angel departs, Ya’akov asks him to reveal his name. The angel replies, “Why do you ask my name?” and leaves without answering. Why did the angel evade Ya’akov's question and not respond?
R’ Leib Chasman explains that the angel did answer Ya’akov’s question. A name defines the essence of a person, a thing, a being. (In the past I’ve quoted the Mei HaShiloach that shem = ratzon in gematriya; your aspiration, your ratzon, is the essence of who you are.) The angel who battled Ya’akov was the embodiment of the yetzer ha’ra, Eisav’s protector. The yetzer ha’ra has no true essence and indentity – it is the power of illusion, the temptation of chimerical vision. Wrongheaded decisions, acquiescence to temptation, incorrect goals, are all products of our being tricked into thinking there is some other path, some other reality, other than what Hashem wants and decrees. Focus on the truth, the real essence, and the yetzer has no power.
Perhaps this ambiguity -- the lack of name -- of the angel's essence has to do with the machlokes (Chulin 91) we discussed earlier as to whether the angel appeared to Ya'akov as a pagan or as a talmid chacham. The yetzer hara employs different strategies to suit his needs, and therefore is often hard to identify.