Just like how many female characters are defined by their relationship to the default or already established male character, many distaff counterparts are defined primarily by their relationship to the male counterpart. There's also the third kind of Spin-Off, with a long-lost sister showing up or a new girlfriend brought in, only to repeat whatever circumstances gave the main character his powers/duties/what-have-you. While heroes are usually the ones to get this kind of treatment, it's been happening more and more to villains too. This method is very popular among action-adventure series, which mostly appeal to young males. This allows the station or network to fill up extra space on the schedule with a known quantity, while (in theory) stretching its demographic to female viewers. One common method used in creating a Spin-Off is taking the main character of the show, creating an equivalent that's mostly the same except for being the opposite sex, and then packing the new character off to her own series.
