Bro said he wanted to become a girl in his previous life, so she's basically a trans women boymodding as a pretty dude. Now that she's a full blown girl with her desired equipment, ig she's now a cis women swinging both ways for the sake of NTR
Kanajo doesn't nessecarily means girlfriend. Could also be used as a pronoun in Japan.
Instead of "one day, my gf became a girl" it'll be "she turned into a girl one day"