I really like Sei. He is such a wise and smart omega, with an incredible backstory. I really enjoyed reading about his struggle to fall for his first alpha and how he and Ma chan gradually grew...closer, I guess?
The manga is actually awesome, it's a lot of fun to read an omegaverse with a character who isn't instantly falling in love. The romance didn't convince me much, but Sei's personality and character was such a joy to read about that I give it 5*
Omega-sama Night Club