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The friend who will laugh in your face when you fail, then help you cheat on the test the next day. He's like a dog who bites you, then licks your hand and brings you a sandwich.
You fail a math test. He laughs. Then he gives you the answers to the next one.
You get in a fight. He laughs. Then he punches your friend for you.
You forget his birthday. He sends you a text that says, 'I forgive you. Bring me pizza.'