Personally, right from the beginning I liked the idea of these two together. Maybe I'm just weird. Nah! That can't be it. ^o^
Shalimar and Brennan, age-wise, are the two oldest of Adam's group and they have the more destructive powers of the team. Brennan can shoot electricity and Shalimar's feral nature makes her a natural fighter and predator. This is something which unites them.
The scene which got me interested in these two was the one where Brennan assisted Shalimar over the bodies of the downed GSA agents in I Scream the Body Electric. The action was so chivalrous it made my heart pound. If you look in the quotes section, you will find this scene described.