The Picaro class is designed to emulate characters like the Grey Mouser who are formidable in combat, but unlike D&D fighters tend to spurn armour and other such accoutrements. These type of characters also tend to have some magical aptitude as well which can be tricky to fit in with the D&D style of niche protection with fighters, magic users and clerics, though the cleric class does allow a mixture of melee ability and spell casting.
I'll not try and stat out the class for any particular system yet, but in Swords & Wizardry, Labyrinth Lord or any of the other D&D style games out there it will be broadly similar. Weapons somewhat limited to the more characteristic ones for this sort of character of swords, daggers, ranged weapons. Armour limited to lighter types such as leather or chain shirts rather than full suits of armour. Magic use would need to be fairly gradually introduced as these types of characters are primarily melee fighters rather than spell casters.