Rhadamanthine and adamant are synonyms for stern. Genial is the opposite of stern. We have given below a collection of synonyms for "stern".
The best word for that look is stern, which means "tough" or "serious." Stern, strict, severe, harsh, unforgiving—they all mean more or less the same thing, which is very hard and precise, with a little helping of seriousness given for good measure. For example, when Charles gave her a stern look and she made a face. He gave her a harsh glare that made him smile. Matilda seemed ready to refuse, but Miley pushed her hard.