Harbinger : One that indicates or foreshadows what is to come
Miser : a person who hoards wealth and spends as little money as possible.
Ascetic : A person who renounces material comforts and leads a life of austere self-discipline, especially as an act of religious devotion
Follower : a person who supports and admires a particular person or set of ideas
Counselor : A person who gives counsel, an adviser
Braggart : a person who boasts about their achievements or possessions
So a harbinger sees what is to come and in opposite a follower admires his idea.
Hence, antonym of Harbinger is Follower.