During Tanjiro Kamado's trial, everyone was surprised that Tomioka Giyuu was releasing protective pheromones while protecting the young Kamado from Sanemi an Alpha who intended to hurt Nezuko, so there was no way for him to let him hurt them.

Shinobu: Ara Ara Tomioka-san Why didn't you tell me you were an omega instead of beta said beta

Mitsuki: Tomioka-san protects them as if they were his little ones how cute, the beta thought

Tomioka: Yes I am an omega, he admitted.

Tanjiro: Damn, Tomioka's secret was revealed by my fault, he thought.

They were all surprised by what the jet had just admitted, they had never imagined that Giyuu was an omega.

Kagaya: My children, what's going on?

The girls who accompanied Kagaya told him about the situation.

Kagaya: I see it had to happen one day, Giyuu after the trial you will have to explain why.

Tomioka: Yes, Master.

After having settled the business of the kamado and that the stealth ones took them to the domains of the butterflies to treat their wounds.

Tomioka: The reason is that the omegas are considered weak by the organization, that we never had them within the organization that's why I hid it, because I wanted to avoid problems..

When Rengoku learned that Giyuu was an Omega, he existed a little, because he had developed feelings towards the jet, but he will have to be careful he was not the only one who wanted to mark him as his own and to conquer it