Pearl Fey is a powerful candidate, since she's saved the day quite a few times by channeling Mia.
For someone who is ostensibly the series' main Distressed Damsel, Maya certainly makes herself useful under the direst situations.
An evil example of this trope would be Dahlia Hawthorne. Not everyone has the guts to jump from a bridge into a river known for its deadly speed just to steal a jewel.
Arguably, Ambassador Colias Palaeno from Investigations, who manages to really grow on a lot of players, and who shows up with a small army near the end and pressures the Big Bad into staying put with thinly veiled threats. And he keeps smiling while doing it.
We have Rena, whose primary prerogative on a normal day is to find cute things to take home. When pushed to protect herself or her friends, she can non-fatally swing around a cleaver like no one else.
Satoko, appropriately dubbed 'The Trap Master' by the rest of the cast, is somewhere between 9 and 12 years old and very frequently The Woobie, yet she can trap, confuse or outright take out trained, fully grown men with minimal effort on her part.
Finally, we have Hanyuu, a little girl around Rika's age, cute enough to be kidnapped by Rena three times in two minutes, who is also Oyashiro-sama, a Physical God who is promoted from The Voice to a full cast member during the final arc.
Veridadia from Yumina The Ethereal, who can turn herself into a BFS capable of destroying anything in a single strike.