If "monster" refers to a creature:
* Human: Emphasizing the contrast between a terrifying being and a normal person.
* Angel: Suggesting a benevolent, ethereal being.
* Fairy: Conveying a sense of small, magical, and harmless creature.
* Animal: Highlighting the difference between a mythical or grotesque being and a regular animal.
If "monster" refers to a person with cruel or evil qualities:
* Saint: Implying someone with exceptional goodness and virtue.
* Hero: Signifying someone with courage, strength, and noble intentions.
* Benefactor: Suggesting someone who does good deeds and helps others.
* Altruist: Emphasizing someone who acts selflessly and for the benefit of others.
If "monster" refers to something exceptionally large or powerful:
* Tiny: Contrasting with the monstrous size.
* Weak: Highlighting the opposite of monstrous power.
* Unremarkable: Indicating something average and insignificant.
The best antonym will depend on the specific context and the meaning you want to convey.