The Lost Souls in Oninaki are basically side quests that you can find throughout the world. Some of them are easy to find, and other can be hidden and hard to spot. Check out this Oninaki Lost Souls guide to complete them all.
Oninaki Lost Souls Guide
You can’t complete this until you get the Daemon, Will. He uses an axe and when you have him equipped speak to the boy. This will complete this quest. I had the Kumbhakarna axe equipped, not sure if that was needed or not.
Return to the town of Deto and to the West of the #2 waypoint you can find a woman on a bench. When you approach the quest will automatically complete.
When you talk to the soul it will turn into a beast. Defeat the beast and you will beat the quest.
This is another one where you have to fight the guy after speaking with him. Kill him and complete the quest.
This is another one where you have to fight the soul, kill it to beat the quest.
She wants to be taken to the ground floor of the Ehir Palace. Enter the Ehir palace and go to the left of the stairs. Cross the veil there and the woman will thank you and depart.
You can find the Mesid Woods in the Mazzen Mountains. Warp to way point one and follow the path until you hit a house, the quest will complete there.
from GamersHeroes http://www.gamersheroes.com/game-guides/oninaki-lost-souls-guide/