Metheri is a paladin because her mom was a paladin. Courtney Brighthelm was one of the most famous paladins of all time in Azul. Brighthelm was responsible for order on the offshore islands due to a personal crusade to defeat the brigands who terrorized the islands. Brighthelm organized the locals, fought the pirates, installed local sheriffs to maintain order, and even led an attack on the pirate base. The result of all this was that Metheri had some big shoes to fill.
Metheri also had the world's best tutor. Metheri got her first weapon when she was 4. She learned to ride horses before she was 6. She had even won her first jousting tournament when she was 9. Metheri was too good at what she did that she was soon noticed by the local military and was drafted the day she turned 12.
Being drafted was a shock to Metheri as well has her mom. Metheri would now be trained by others who were sometimes less trained than Metheri. Also, being a paladin put her at odds with some of the military higher ups. Metheri had also learned her moral lessons well and would not put up with any 'gray'.
Several times Metheri had turned in superiors for breaking military and civil law. Metheri could not see how her zealous approach to law and right where setting her up for failure in the military. Or maybe she did see this and did not care. Metheri is a woman of character and would sacrifice everything to do what is right and just. Her mom would have been proud.
This friction with her superiors is why she was sent on the mission to deliver the note to Duke Rudrik of Alban.