Serio would be the correct answer because Pablo (noun) is a male, and this sentence is singular. Now for a female the correct adjective would be seria. Remember all nouns have a gender whether it be a place, or thing.
2.) Originally La Llorona and El y La luna are a legend but since it does not come out you can say that it is a myth // Originalmente La Llorona y El y La luna son una leyenda pero como no sale se puede decir que es un mito
