Antonyms for the word "is lenient" are "is strict," "is rigid," "is severe," "is harsh," "is demanding," "is intolerant," and "is inflexible." A lenient person tends to be gentle, showing mercy, and being forgiving. On the other hand, a strict or rigid person tends to be tough, firm, and often unsympathetic in enforcing rules and regulations. A severe or harsh person could be excessively tough, and they may not consider extenuating circumstances when enforcing rules. A demanding person requires high standards and is not satisfied with anything less than perfection. Finally, an inflexible person is unyielding and unwilling to compromise.