Loki was a mischievous prankster and spy amongst the Aesir.  He took many forms to hide or trick others, mortal and immortal alike.  No one was safe from his prying eyes and dangerous wit.

“Harm seldom befalls the wary; 
With his hears listens, 
And with his eyes observes: 
So explores every prudent man.”

-- The Poetic Eddas, The High One’s Lay