Alexei is the husband to Sachi (A human woman) and the father of Yuliy and Mikhail.
Before the vampire attack, Alexei was born as a (royal) member of the Sirius tribe. Eventually, he met and married a human woman named Sachi, whom the Sirius accepted. They had two sons named Mikhail and Yuliy. (The series never explains how Sachi encountered the Sirius tribe village, which was meant to be hidden from humans.)
The story of Alexei past was revealed in episode 10 where Alexei met Akasaka. The history of the Sirius origin was briefly explain by the Gustav, one of the Sirius elder to Alexei in the hope of teaching him how to seal the Ark of Sirius. The responsibility and secrets of the Ark of Sirius was pass down to Alexei and was sealed by him at the cost of his own life.
Later when Yuliy was searching for the Ark of Sirius, he met Alexei at Sirius Secret Chamber where the Ark of Sirius was seal together with Alexei soul.