Sōichirō Nahashi, who is always referred to by his surname (as per Japanese standard), is the guardian of Lucia, and he loves her as if he were her real father. Before Lucia was born, Nahashi was studying to become a mage, along with Lucia's mother Lilith and Lucia's father Lucif.
He harbored an unrequited love for Lilith when they studied together, but never got close with her as she was in love with Lucif. He cares about Lucia very much, and does whatever he can to help her. He also cares about Sumire, and tries to help her control her beserker side, hoping that she may be able to fix Lucia instead of trying to break her.
Nahashi is an older gentleman with grey hair that goes just pass his shoulder, a mustache and goatee. He wears tiny glasses, and has grey eyes. The tiny glasses he wears allow him to see viruses.
We meet Nahashi in the very first chapter of the manga, when Sumire comes to the shop and Lucia introduces her to him (quietly) as "the perfect bait."
In Chapter 22, Nahashi first meets Lilith and introduces himself as Soichiro Sakakizuka, not Soichiro Nahashi. In that chapter we also find out about Nahashi's glasses, which enable him to see viruses, despite not being born with the sight.
In chapter 37 it is found out that Nahashi is Lucia's true father the reason though never being mentioned