Antonyms of mischievous :
sober, serious, harmless, unplayful, advantageous, favorable, helpful, profitable, serviceable, beneficent, good, invigorating, rejuvenating, useful, beneficial, healthful, life-giving, salutary, wholesome, grave, grim, sedate, solemn, staid, stern, acquiescent, compliant, complying, dutiful, obedient, submissive, clean, correct, decent, decorous, moral, proper, respectable, considerate, courteous, kindly, mannerly, polite, thoughtful, angelic, cherubic, divine, heavenly, amenable, docile, governable, tractable, amiable, complaisant, good-natured, obliging, pleasant, discreet, modest, adult, grown-up, mature, behaved, behaving, nice, orderly, propitious, curative, healthy, palliative, remedial, salubrious, secure, sound, benignant, noncorrosive, nondestructive, nonfatal, noninfectious, nonlethal, nonpoisonous, nonpolluting, nontoxic, nonvenomous, anodyne, benign, hurtless, innocent, innocuous, inoffensive, safe.
Examples of usage:
And when we had remained there a few dayes, I understood of a wicked and mischievous fact committed there, which I have put in writing to the end you may know the same. "The Golden Asse", Lucius Apuleius.
Should they not immediately cease from a course mischievous in every stage, and finally tending to the greatest catastrophe? "The Rise and Fall of the Confederate Government, Vol. 1 (of 2)", Jefferson Davis.
After me, my masters, continued Matthew, who was still the leader of this insane band of mischievous fools, after me. "The Tangled Skein", Emmuska Orczy, Baroness Orczy.