Steve Rogers/Captain America: Your best friend is Sharon Carter/Agent 13.
Tony Stark/Iron Man: Natasha Romanoff is your best friend
Bruce Banner/Hulk: Your best friend is weirdly enough Tony Stark.
Thor: You happen to four best friends Lady Sif and the Warriors Three.
Loki: Your best friend to Loki chagrin is Clint Barton/Hawkeye the guy he mind controlled that one time.
Clint Barton/Hawkeye: Your best friend is Agent Phil Coulson
Sam Wilson/Falcon: Your best friend is Darcy Lewis
Pietro Maximoff/Quicksilver: Your best friend happens to be his twin sister Wanda Maximoff/Scarlet Witch.
Stephen Strange/Dr. Strange: Most people would think your best friend was your sister Christine but you are actually best friends with Wong.
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There is another new update for this story. I hope you enjoyed it. Please feel free to leave a request.