noun
stubborn person; diehard
Describes a person who is obstinate and refuses to change their mind or ways. Often carries a negative connotation of inflexibility.
彼は一徹者で、一度決めたら絶対に曲げない。
He is a stubborn person; once he decides something, he never backs down.
あの一徹者を説得するのは無理だ。
It's impossible to persuade that diehard.
一徹 is a na-adjective meaning 'stubborn' or 'obstinate'; 一徹者 is the noun form referring to such a person.
Compound of 一徹 (いってつ, 'stubborn') and 者 (もの, 'person'). The exact origin of 一徹 is uncertain, but it is used to describe someone who sticks firmly to one way.