strict
lenient
Lax or lenient.
The opposite could be strict (not lenient), or harsh (the reverse of lenient).
lenient,liberal
The answer to the analogy "rigid is to flexible" is "strict is to lenient." Just as rigid and flexible are opposite in terms of stiffness, strict and lenient are opposite in terms of severity.
The judge's decision was lenient, she was only given a fine.
lenient
laxLax, gentle, loose, liberal are all opposites of strict.
easygoing, forgiving, indulgent
There is no formal word unsevere to mean "not severe." Words that mean the opposite of severe are mild, gentle, and temperate (for people: friendly, tolerant, or lenient). The word unsevere would be a case of using a known, specific word simply to suggest a reverse meaning, such as unlarge or unhated.
The noun form is lenience, the subjective noun for relaxation or compassion in applying discipline or punishment.
· lefty · legendary · lenient · lethargic