How about 'Sedate'?
Smooth
The opposite of visible is invisible, meaning unable to be seen.
The most accurate opposite of retract is protract
The opposite of the word "cool" is typically considered to be "uncool" or "unpopular."
'Silence' could be used for the purpose, but human language does not really have a word for the opposite of 'echo', just as silence is not the opposite of sound (merely its absence). However, if one considers an echo to have periodicity, we could consider something incessant, such as "din" to be its opposite. However, it is only opposite in a shade of meaning.
minimul
Bare
Servant
Defeated
unethical.
savant.
dawdle
Smooth
This word does not have an opposite.
There is no antonym for the word avenge, although there is an opposite for the meaning. The meaning is revenge, so the opposite meaning would be forgiveness.
Cowardice is nearly the opposite in meaning of valor.
what word is opposite of prudent