Honestly, I begin to like that imaginary friend idea. First of all it reminds me of Terry Pratchett, because there is a hermit with an imaginary friend, who truly exists. (and saves the hermits life), and second of all it avoids many stupid, silly things that can happen.
Hermit in hospital... too bad, no one can bring him food/water/crutch.
Hermit dodged on top of the wall... too bad, no one can build stairs for him.
Hermit dies by instakill/crazy/fire/something... imaginary friend has ability to revive him, sacrifing himself. Like a second chance, giving the player ONE mistake they might survive.
It also means you have twice the workforce.... could be a mid-game ability.
I might add some form of full-heal as well. People lose limbs too often. Maybe a health potion from the alchemist. something you can store away. That way he might drink it, even if he is too banged up to run reactions.
Can there be a way for him to learn other skills? Or at least practice them? Also, how do you know which one will survive, so you can assign skills on embark?
Other skills? He can learn any skill there is, just like any normal dwarf. Practice I have for combat skills atm. Social skills are probably not that important for a hermit. The rest you learn by doing. Medical skills are also pointless, he cant doctor on himself.
You dont know which one will survive. You can either give all of them the same skills (costing points), or give them none. When I said that he has natural skills, I wasnt exaggerating.
[NATURAL_SKILL:MASONRY:5]
[NATURAL_SKILL:CARPENTRY:5]
[NATURAL_SKILL:DETAILSTONE:5]
[NATURAL_SKILL:TRAPPING:5]
[NATURAL_SKILL:TANNER:5]
[NATURAL_SKILL:BREWING:5]
[NATURAL_SKILL:COOK:5]
[NATURAL_SKILL:HERBALISM:5]
[NATURAL_SKILL:WOODCRAFT:5]
[NATURAL_SKILL:STONECRAFT:5]
[NATURAL_SKILL:LEATHERWORK:5]
[NATURAL_SKILL:BONECARVE:5]
[NATURAL_SKILL:ARMOR:5]
[NATURAL_SKILL:BOWYER:5]
[NATURAL_SKILL:BOW:5]
[NATURAL_SKILL:SNEAK:5]
[NATURAL_SKILL:TRACKING:5]
[NATURAL_SKILL:KNAPPING:5]
[NATURAL_SKILL:DISCIPLINE:10]
[NATURAL_SKILL:KNOWLEDGE_ACQUISITION:10]
[NATURAL_SKILL:SITUATIONAL_AWARENESS:10]
[NATURAL_SKILL:CONCENTRATION:10]
[NATURAL_SKILL:COORDINATION:10]
[NATURAL_SKILL:RANGED_COMBAT:5]
[NATURAL_SKILL:MELEE_COMBAT:5]
[NATURAL_SKILL:DODGING:5]
[NATURAL_SKILL:STANCE_STRIKE:5]
[NATURAL_SKILL:GRASP_STRIKE:5]
[NATURAL_SKILL:BEEKEEPING:5]
[NATURAL_SKILL:PLANT:5]
Edit: Just remembered maxage. Deleted it, no death by old age for hermits. That would be anticlimactic.