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Loki is by-birth a blue frost giant. A frost giant's appearance isn't like that of Asgardians/Humans.

In Thor movie, Casket of Ancient Winter was able to transform Loki to original form temporarily. And, Loki was surprised with this. Meaning, Loki wasn't actively transforming his appearance using his imagery tricks. Plus, his imagery tricks don't work when his concentration is lost. But yet, we can see his Asgardian appearance everytime. Means, it's not his imagery trick.

How is Loki's default appearance like Asgardians/Humans?

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    Specifically, Keen's answer addresses this. – phantom42 Apr 24 '14 at 17:09
  • @phantom42 US military will reach the secret base... You know where. – Baby Yoda Apr 26 '14 at 0:10