The phrase "most light-minded" suggests a person who is carefree, easygoing, and not very serious. The antonyms for this phrase would imply someone who is more serious or focused. Words like sensible, grave, earnest, and solemn may be used to describe an opposite personality type. These individuals tend to take things more seriously, are not easily distracted, and are more thoughtful in their actions. Other possible antonyms for "most light-minded" could include intense, driven, driven, and passionate. Ultimately, the opposite of being light-minded would be someone who is serious-minded and deeply focused on their goals and duties.