“Eh, where's everyone going? What? We're finally marching on Zofia Castle? Then I'm going too. Let's go for it -- for the liberation of Zofia!”
—Paison in Gaiden
Paison (パイソン, Pay or Python in the fan translations) is from Fire Emblem Gaiden. He guards the Zofia Liberation Army camp headquarters, and works directly under Clive. After the war, he became the leader of a vigilante group and protected villages from bandits.
The player first meets Paison at the entrance of the Liberation Army's HQ, but must fight your way though the army of the undead within the building to talk with Clive and Fols first. On your way out, if you talk to Paison again, he will join you.
Paison is a glass cannon. He is the only Archer you can recruit in Alm's route unless you promote one of the Villagers to Archer. Archers in Gaiden are great units since they can not only shoot 3 spaces away (or 5 with a weapon equipped), but they can counterattack units next to them or at their max distance. The good news is that Paison has high Strength and will be able to dish out huge physical damage from far away. The bad news is that his Speed starts out very low and his Speed growth is average, which means it is doubtful that he will be Double Attacking often which, hurts his offensive capability. If Paison were just a little faster, he would be one of the most dangerous units. As it is, he still makes up a solid part of the team and will round out your team well as long as you keep him well protected.
The only other non-Villager Archer in the game, Leo, is not as good as Paison. Leo has significantly lower Strength and even worse Speed. He has better Defense and HP, but this is not very useful as Archer is mostly a ranged offensive class that should not be getting physically hit or counterattacked that often.
In order to protect villages from plundering bandits, he put all his effort into serving as the captain of a vigilante group.
“Tsk, my bad...”
—Paison's death quote in Gaiden.