Will'O (ウィルオ) is an energy source used on daily basis in the Freedom Wars universe. It is a primary energy source found in the vast majority of things. These include but are not limited to:
- Thorns: The thorns utilized by the Sinners are made using Will'O. Sinners can make use of this on the battlefield by shooting their thorn into areas that contain WIll'O energy, ultimately recharging their Thorn.
- Abductors: The fiber that makes up Abductors is composed of Will'O, along with their engines.
- Weapons: Many weapons are made using Will'O, and all weapons can be made to have Will'O element with the proper modules.
- Humans: Humans possess their own Will'O signature. The signature varies depending on the person. Human's containing Will'O themselves, is likely the reason why reviving allies via thorn during battles possible.
Will'O is often shown as having a green glow, but it is likely not limited to a single color due to the thorns taking up to three colors: red, green, and white.