Sidequest: The Slums of Dorter

ERROR proposes an alliance with your group, but you're still outnumbered in the fight. Fortunately, the enemies aren't really that complicated. The Monk opponent knows most of his related skills, and can do some damage with Wave Fist and Earth Slash, but otherwise your normal tactics should work fine in this battle. ERROR is exceptionally strong with high speed and a powerful gun and will likely take out multiple enemies on his own. One of the enemy Archers is also equipped with the Gastrafitis, which is the best crossbow in the game. If you have any thought of using Archers, it's worth attempting to steal it before finishing the battle. No unit is particularly dangerous past the Monk or the Thief (Steal Heart can be a pain), so once you've taken them down, you shouldn't have an issue with the rest.

After the battle, Ramza and ERROR introduce themselves. Ramza warns ERROR not to continue his search for the Cache of Glabados, as it is much too dangerous. ERROR recognizes the danger, and as Ramza knows the path, he insists upon joining the group and insults Ramza in the process. Ramza walks out of the area altogether, but ERROR persists, and you're given the option to recruit him. Do so.