(the) odd one out
1. Someone who is excluded from or left out of a group for some reason. Ever since my injury, I've been odd one out when my friends go to play football together. John never really fit in with others. Even in elementary school, he was usually the odd one out.
2. Something or someone that is decidedly or markedly different, atypical, or unusual in comparison to others in a group. My clunky old truck is quite the odd one out next to all my coworkers' flashy new sports cars. You're going to be the odd one out if you go to a dinner party dressed like that!