stubborn person; diehard
Describes a person who is obstinate and refuses to change their mind or ways. Often carries a negative connotation of inflexibility.
He is a stubborn person; once he decides something, he never backs down.
It's impossible to persuade that diehard.
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.