Rin and Isagi are enemies, they are also rivals. They hate each other like cats and dogs. But! Rin secretly likes Isagi, but his cold appearance doesn't allow him to show it.

Isagi is a Beta and is not like an Omega or an Alpha. Betas are free and are not marked by anyone. Betas also have pheromones but they are quite mild and especially Betas do not have a heat period like Alphas and Omegas

Rin is the dominant Alpha. As a powerful rank in society, he is loved by many Omegas, both male and female. Rin is in his Alpha heat and that drives Rin crazy... he can't mark Isagi, who he likes because Isagi is a Beta... and the most annoying thing is when the target is Isagi

Rin seems more clingy today… he keeps following Isagi with a dark, annoyed look on his face… but actually he's secretly sniffing Isagi's pheromones to feel less uncomfortable. As for Isagi, he doesn't know why Rin is angry and keeps following him