Ducky isn't a fighter, and neither is her mother. [...]
Seriously, though, it is hard to imagine Ducky growing up to be anything but a mother who is at least as good as the one who is raising her in the films...
Oops. Subversion in
Twilight Valley! Every character gets in on the action... even Ducky! Yep, yep, yep!
Only thing weirder than Kung Fu Ducky is...
Ducky's mother as an Escrima fighter and Ducky as a "Krav Dino" fighter! (See
Twilight Valley <link in my sig> for more details)
Seriously though, as much as I think "damsel in distress" is a MAJORLY overused trope, if you have to use that trope, Ducky is perfect.
Cera can't serve that role. She could serve as the "woman warrior" trope. She's quite strong and a dangerous foe against enemies her size. Horn + body part = bad.
Petrie can't serve that role. He's male and is flight-capable.
Ruby can't serve that role (as Kor pointed out, she'd probably punch out her captor!

)
At least in LBT 7, Ducky was capable of escaping her captors without assistance (temporarly at least).