Sebastian is a servant of the Phantomhive family, his current master is Ciel Phantomhive. 18 year old kid, he just got labeled an omega at the age of 18, that's quite a long time.

during the first period of ciel's mating cycle, ciel feels so hot. he kept moaning and complaining to sebastian, sebastian is an alpha, which alpha smells strongly of another omega? moreover, Ciel's pheromone scent is also very sweet