na-adjective, noun which may take the genitive case particle 'no', noun
obstinate; stubborn; inflexible
Describes a person or attitude that is unyielding and determined, often with a nuance of being headstrong or uncompromising. Can be used both positively (steadfast) and negatively (pigheaded).
彼は一徹な性格で、一度決めたら絶対に曲げない。
He has an obstinate personality and never budges once he's decided something.
一
An inflexible attitude sometimes causes clashes with those around you.
Composed of 一 (one) and 徹 (penetrate, thorough). The combination suggests a single-minded, thoroughgoing persistence. The exact historical derivation is uncertain, but the kanji meaning aligns with the sense of unwavering determination.