Tony
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Steve: you were only 4 at the time and often got words mixed up so you called him Caping. And it just sorta stuck.
Bruce
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(Agent Hill) Maria: she always gave you little sweets our chocolate buttons so you called her sweetie, you still call her that now.
Steve
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Clint: you were watching the hunger games one day so you called him Katniss, you don't really call him that these days unless your upset.
Thor
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Loki: King of Mischief. he found it funny and always tells you that you were right in calling him king.
Clint
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Phil: you called him Prince Phillip because you favourite film was Sleeping beauty.