fellow fan of the same idol/character
Slang used in idol and character fandoms to refer to someone who supports the same member or character as oneself. Often used in phrases like 同担拒否 (rejecting fellow fans of the same member).
I was happy to find a fellow fan of the same member on social media.
Be careful because some people reject fellow fans of the same member.
Abbreviation of 同じ担当 (same person in charge/favorite member), from idol fandom slang where 担当 refers to one's favorite member.