Meet Greg. Greg was born with pretty average genetics. It seems like he was destined to have average intelligence, average strength, and be average at everything. There was one particularly though: he has unlimited willpower.
Willpower is the ability to resist short-term temptations in order to meet long-term goals. Having unlimited amounts of it, Greg can, for example, withstand any amount of torture (although he will still suffer trauma), exercise until his body collapses of exhaustion, refrain from eating until he faints, or
even hold his breath until he passes out (actually, holding your breath before you pass out may be prevented by subconscious mechanisms, so nevermind), if he so chooses.
Of course, these are just the most extreme though not very useful examples of his powers. More practicality, he can break any bad habit or begin any good habit by just deciding to do so. He is immune to addiction. He can do something boring for any amount of time he chooses, such as learning about something he's not particularly interested in, exercise for any amount of time he chooses (if physically able), follow any code of conduct to the best of his physical ability, etc...
This of course allows him to be a superhero/supervillain (depending on his goals) similar to batman. He would likely be super intelligent, super fit and strong, have lots of skills including leadership, etc..., since these would likely help him advance his long term goals.
My question is, what is the most plausible way Greg could exist? It could be either natural or artificial.
I think the most plausible answer is some sort of medication that completely suppresses one's desire to complete short term goals instead of long term goals. Another possibility would be that Greg was born with some brain deformity to the same effect. This has the interesting side effect that when Greg takes the medication/when he is born, he may completely disregard things like food or water (which would obviously be life-threatening) until he figures out that he needs to "will" himself into eating, drinking, sleeping, and going to the bathroom, since he no longer is compelled to.
P.S. If you use Greg in a story, he could very easily become a Mary Sue. To mitigate this, some possible weaknesses would be stubbornness, odd beliefs or delusions, or obsessions (such as revenge). Of course Greg could get rid of these weaknesses if he wanted to, but the idea is that he probably wouldn't. Another weakness is that if you someone how suppress his superpower (for example, by cutting off his medication), he gets extreme avolition.