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This achievement ties in with "Bold! Dramatic! Heroic!". The character customization location is in the Outer Municiberg District inside Edna Mode's house. You can fast travel to her house with your map.

You need to unlock a character with a cape so wait until you've unlocked Reflux by completing Chapter 2, 'Elastigirl on the Case', in the story. Just stand on the glowing zone once inside her basement and make a character with a cape. Give the character animal control so you can use it for hub collecting later if unlocked.