sorry for poor english >__<;;

if u check heroic soul, they have preferred weapon type. it same as weapon they can hold in mainhand. warrior soul can use axe and sword, knight soul can use sword and spear, healer soul can use staff and spear and so on.

u can equip any weapon type in 9 sub slot. but if it same as soul preferred weapon type, it add 20% extra HP and ATK.

example- if u have 1000ATK 100HP SSR spear

u equip mainhand on Olivier, it give 1200ATK, 120HP
u cannot equip mainhand spear on Merlin

u equip in sub weapon slot on Olivier, it give 1200ATK, 120HP because it preferred weapon.
if sub weapon slot on Merlin, it give 1000ATK, 100HP because it not preferred weapon.

we did testing and provided proof that this mechanic exists in Kamihime Project in the FAQ thread.