The 63

Rule 63: If there's a character with a gender, it can be swapped.

The character who's essentially another one but with the opposite gender. If there's a girl, a guy will be her 63. If there's a guy, a girl will be his 63. The 63 is an oddly recurring character role who has strong physical resemblance to another character, likely someone from the Main Cast, and does not necessarily need to have a similar personality to their un-63 counterpart.

Contextually, the 63 is related to the Modified, Family Member Clone or Alternate Timeline Clone (albeit less so than a Modified), and Evil Twin as they are all another character with certain changes to them.