Make sure it's Advanced editor.
First, make a variable title 'gender' or something like that.
Next, have an option where you chose the gender.
If they choose Girl, then add 2 to gender. If it's boy, add 1 to gender.
Then, on the pages where they share the same story line, instead of 'he' or 'she' put in something like this;
%%GENDER%=%1%he%%%%GENDER%=%2%she%%
Or, if you decide to make it two varibles, have it start at zero and add 1 to which ever gender they choose.
%%MALE%=%1%he%%%%FEMALE%=%1%she%%
To access the variables changed by a link, open the page, and click the little sign that looks like a '+' with and underscore beneath it.
