Ardent Fierce in Chinese / Japanese...

烈 is a Chinese word that means ardent; intense; fierce; stern; upright; to give one's life for a noble cause.

In another context, this character can refer to one's exploits or achievements.

In the Buddhist context, this is burning, fierce, virtuous, and/or heroic.

While technically, it had the same meaning in Japanese, it's usually a female given name, Retsu in Japanese these days.

貞烈 is the Japanese Kanji for “Extreme Faithfulness.”

The first Kanji means “firm adherence to one's principles,” chastity (of a woman), chaste, etc.

The second Kanji means ardent, intense, fierce, stern, upright, to give one's life for a noble cause, exploits, achievements, virtuous, and in some contexts, heroic.

Now you get the idea why this refers to someone who is extremely faithful (to a cause, themselves, their religious beliefs, or their philosophy.