By definition, someone who does not agree with anything would be disagreeable. Though argumentative may also describe such a person, depending on the situation.
There is a southern tradition of this. Some people just will argue any point, and in southern culture we refer to them as a "Contrary." a Contrary will argue his own name with you, will argue that water ain't wet and sugar ain't sweet.
There's no rhyme nor reason to it; they're simply born that way. Any discussion with a Contrary will eventually turn into a debate.
A person who is never satisfied with anything can be described as being discontent, perpetually dissatisfied, or perpetually unsatisfied.
An optimistic person.
A person who studies philosophy is called a philosopher.
A person from the Philippines is called a Filipino.
A sage.
anything done to a person that they did not agree to is considered abuse.
You shouldn't call them anything.
collector
Suggesting that you cannot know anything is saying that you know something(that you know that you know nothing). Lets call it a draw.
Such a person could be said to be defeated.
A person who is never satisfied with anything can be described as being discontent, perpetually dissatisfied, or perpetually unsatisfied.
benevolent person
Naive.
a dead one
A person with a bad temper is called "enojon"I agree but also.....A person with bad temper is also called temper active.
Mr or Mrs Google!
impulsive or rash In general, I agree with Gonzo, above. But whether or not a person can do this effectively does make a difference. I would call a person who can make effective quick decisions a leader. This probably wouldn't be her/his preferred mode.