Something knocked out fairly rapidly when I had a moment of inspiration.
Link-boys and lantern-girls are pretty commonly hired as retainers; kids expected to follow the PCs to provide light, carry things, and not much else. What if you wanted to play as this unfortunate urchin, though? Well, here's how.
Hit Dice: D4
Saves: As Cleric
Attacks: to-hit as Cleric
XP to level up: half of that required for a fighter.
Night Vision: A lantern-girl is accustomed to carrying the light for the party. She can see twice as far by any light from a source she herself carries.
Perceptive: A Lantern-girl has a 3-in-6 chance to hear noises, to notice unusual architecture, to be woken by the sounds of intruders, and so forth. She is only surprised 1-in-6 of the time.
Insignificant: Many monsters and NPCs will assume that - being only a child - the lantern-girl is unimportant and no threat to them, and act accordingly. This may mean that monsters do not attack her while there or other valid targets. It may also mean that human NPCs treat her like a mere child, even when she's accumulated quite a lot of wealth and experience. This ability is as likely to hinder as to help, and is at the GM's discretion.
Fortunate: Being small and nimble, a lantern girl is adept at avoiding perils, and furthermore her childlike innocence (or deliberate facade thereof) can sometimes pause the hand of even the most ruthless enemy. Wherever a saving throw reduces some danger when passed (such as a Save vs Breath halving the damage taken from a fireball), the Lantern-girl instead ignores the danger, and is entirely unaffected.
Sneaky: A lantern-girl can easily escape notice. In an urban environment, she can blend into the urban landscape, becoming just another unremarkable urchin; when she does so, she has a 5-in-6 chance to escape detection from those trying to locate her. Likewise, in the countryside or dungeon, she can attempt to hide behind whatever cover presents itself; here, she has a 2-in-6 chance to escape notice.
A lantern-girl begins with a torch or lamp; a dagger or sling & 10 bullets; 3d6 silver to spend; and whatever her companions feel like giving her.
For systems which use a Prime Requisite, the lantern-girl's is Charisma. She gets +5% XP if her Charisma is 13+, or +10% if it is 16+.
For systems that restrict Weapons & Armour by class, a lantern-girl can wear only leather armour and can carry a shield. She can wield only one-handed melee weapons, and any melee weapon bigger than a knife requires both hands to use. The only ranged weapons she is allowed are thrown weapons and slings. She is treated as a Thief for the purposes of using magic items.
Upon reaching level 9, a Lantern-girl does not get to build a castle or anything like that; she is, after all, only a child. On the other hand, to have reached that level she probably has a rather nice amount of wealth to invest in the projects of others.