Since it's coming up in other threads, this is intended to explain why some people, not necessarily you, think Tarquin has Swordsage tendencies. It covers only the four biggest points.
https://www.giantitp.com/comics/oots0851.html
Counter Charge: Setting Sun 1 Counter
With a quick sidestep, you send a charging opponent sprawling.
Ballista Throw: Setting Sun 6 Strike
You grab your opponent and spin like a top, swinging him around before throwing him at your opponents like a bolt from a ballista.
https://www.giantitp.com/comics/oots0852.html
https://www.giantitp.com/comics/oots0861.html
Disrupting Blow: Diamond Mind 5 Strike
Successful attack causes foe to be unable to act for 1 round.
https://www.giantitp.com/comics/oots0756.html
Wis to AC (many references)
This is how Swordsage fans interpret him stepping aside all the time.
https://www.giantitp.com/comics/oots0876.html
Crushing Weight of the Mountain: Stone Dragon 3 Stance
While you are in this stance, you gain the ability to constrict for 2d6 points of damage +1-1/2 times your Str bonus (if any). You can constrict an opponent that you grapple by making a successful grapple check.
Malack can learn this through the Martial Study and Martial Stance feats, and credits Tarquin for teaching it to him. This is my favorite part because it adds to the story between these two.
EDIT: I've had a theory since long ago that Tarquin has levels of Fighter and/or Monk for the bonus feats. It would help explan why Nale is multiclassed, and it would give him abilities that stretch outside the Swordsage. But it's difficult because it cuts into the highest level maneuvers he can use, and some of those might be good explanations for what's happening. Also, this theory is what made me give up on this project, since my command of every feat in the game is lacking.
This theory could also explain why Tarquin is on the Fighter class shirt.