My save with 100% gallery and items. Currently at 0 humanity in order to trigger a scene so be careful of dying against mob if you don't want to lose all of the items.
Flame Stone = extra damage against plant and undead, including the 1st boss but excluding the 3rd boss.
Lightning Stone = extra damage to wolf and slug, including the 2nd boss.
Dark Stone = extra damage to 3rd boss.
Magician spirit = +50% on ATK and DEF
Soldier spirit = deals extra 3x15% of your damage. Also, it isn't mentioned but there's probably ATK buff too because normal atk deals more damage than magician spirit.
Missionary spirit = HP regen. Not maxed yet
Equipping a spirit will disable continue (use humanity) when you died to a mob so be careful.