Maqqy

Oh, Tressa's definitely a bit of a nice unit throughout the game. Having access to two weapons and the coveted Wind element before subclasses helps her serve as a versatile Guard-Breaker rivaled only by Cyrus (and the Merchant and Scholar don't share weapons or Magic, making the combination even better, as you seem to have figured out already). This gets even more nuts once she earns the command Hired Help, which adds Swords, Axes, Daggers (with bonus status effect chances!), and Light to her already substantial pool, albeit at the cost of a bit of Gold.

But Gold is where she is meant to shine anyway, with her literally looting money off of dead people when changing maps (...I still can't get over that detail ), being able to buy better (and occasionally cheaper!) items off of people than the town's shop (some of which can't just be stolen by Therion), and eventually learning skills that grant a 25% chance to save an item she uses (and still get the effects) and a bonus to Gold earned after battle though her Merchant Class. 

Add in the Donate BP ability to help fuel more focused characters' shenanigans, versatile stats that let her do well in any class you put her in (though she, as a person, isn't exactly fond of being a Thief), and incredible sustainability in glass cannon-type subclasses thanks to Rest (seriously, why is this thing free?) and Sidestep (Physical damage? What's that?), and you've got a very handy character on your hands.

...And then things get actually nuts in the late game. I won't go into detail there since most of the insanity involves spoilers, though.