The phrase "looked sullen" suggests a person who appears unhappy, depressed, or in a bad mood. Antonyms for this phrase might include "bright," "cheerful," "happy," "jovial," "optimistic," and "upbeat." These words convey a sense of positivity and optimism, and could be used to describe someone who seems to be in a good mood or who has a positive outlook on life. Alternatively, other antonyms for "looked sullen" might include "animated," "energetic," "lively," "vibrant," and "convivial." These terms suggest a person who is outgoing and engaging, and who radiates energy and enthusiasm in their interactions with others.