For decades researchers have been looking for an alternative fuel which is both potent and environmentally friendly.
Countless fuels have been tried but none have caught on be it out of impracticallity, limited supply, or whatever other things you can think of that might stop other fuels and energies from being used. Maybe there are gnomes who plot against alternative power because they secretly own the most commonly used ones. Who knows. That's aside from the point though.
High ranking officers of several large organizations and a few independent researchers and developers have found a new power source which is not only powerful and cheap but brings absolutely no harm to the environment aside from when used for certain large machinery.
Various accounts have given this new power source a few different names, but all have had about the same effect.
Curiously, this source of power seems to require a person's voice to be properly used, and it seems that the amount of power brought out varies from person to person.
Although this may seem like an unreliable power source, it has already played an integeral part in saving several towns, cities, and even the world itself.
An example of one such time is when independent researcher Dr. Simon, already experienced in using this power source, used this source to power his giant fighting robot to combat enemy attacks on his home city making use of the vocal ability to tap into said power source by shouting the name his predecessor, Dr. Kamina, had given it, "FIGHTING SPIRIT!".
Indeed, it is "fighting spirit" which has repeatedly aided mecha pilot after mecha pilot in subduing evil doers. From Domon Kashu of Neo Japan with his Shining gundam to even Shinji Ikari of regular(sort of) Japan with his Eva unit 01, fighting spirit is the clean burning energy which is saving the day when most dire.
Also known as "love", you can obtain fighting spirit whenever in need of protection. Just shout and lunge at your enemy at full force