Well, it isn't a matter of personality.
Tobias is Rachel's last cling to her humanity as a woman. Without Tobias, Rachel would never want to have a sense of being a normal again (proof: book 33, the one after the starfish). If they all survive after the war, I doubt Rachel would join the military, considering Tobias.
Rachel is Tobias's desperate cling to his humanity. Without Rachel, Tobias would never want to be human again. This was made clear in the last book, where Tobias threw away whatever humanity was left in him after Rachel's death by living full time as hawk...
So, I guess, they are the perfect combination.