Toudou Sei

About
Sei is the oldest of the 3 adopted by Tsuchiya Wakako. Currently he is the secretary of Tsuchiya Kuon, though he handles a lot more than just administration.

His story is very similar to the other two. His parents died in an accident and he got adopted by Tsuchiya Wakako on a whim. She felt he'd make a good underling to a boss and that is how he was raised. During that time he was something of a brother to Tsuchiya Kuon and Tsuchiya Yuki, trying to protect them from Wakako's perversions and whim. After all of this he stuck with Kuon and Yuki as the three went out to start their own business. Unlike the others, he kept his own surname as he felt it was the last thing they left him.

He is very loyal to Kuon, he doesn't question his commands even if they are detrimental to his health or business. If he does disagree, he tends to take on a more normal/brotherly tone. Like Kuon, he seems to be an exceptionally gifted businessman. Maybe due to this role, he is also allowed to wear more modern clothes.

He plays a major role in Yuki's events where Kuon suspects them of having an affair. Sei does seem to love Yuki, but was happy that she gets to be with the one she loves. When pushed by Kuon on a possible affair, he is angered and nearly lunges at him before being stopped by Yuki. Whilst being accused of the affair does anger him, he does not go against Kuon's order for a threesome as Yuki allows it.

In his own ending he is shown to obey Kuon's wishes of wanting to work which ultimately leads to his collapse. He feels guilty and stays by his bedside the entire time before Kuon finally dies. A joke ending has him sent to the moon and due to his absence the company collapses. Another has him and Kuon foster their feelings for one another.