Mercenary Crossbowmen are young men who have decided to make a living as archers for hire. After gaining some experience about what they're getting themselves into, they've decided that fighting someone up close is too risky, so they specialized on ranged combat. They took up to the crossbow rather than the usual bow, as it's relatively easy to master unlike the other one, and less stressful. They are still seen as beginners within the ranks of veteran archers, as they're still young and have managed only to fire a limited number of shots (in medieval times, the number of shots was pretty much the standard by which archers were ranked).
As young crossbowmen, they come equipped with the usual: leather armor, a crossbow, simple bolts, a shield for some more protection and a melee weapon (a short sword, a dagger or a spear). While their crossbows can help out against heavier opponents, they don't pack the punch of the bigger bows, and their accuracy leaves plenty of room for improving. As they're mainly ranged, their skill in melee leaves a lot to be desired. Overall, they can be considered slightly better than the usual bandits. Despite being mercenaries, they don't ask for any impressive wages, and they are quick learners too. If you, the player, stumble upon them in a tavern, try to consider if it's worthwhile to hire them.