Alexei appears as a middle-aged man with long white hair and the usual light blue Sirius eyes.
Alexei seems to be a big-hearted person who cares for his family very much. He is also shown to be very assertive in expressing his opinions about sirius tribe traditions.
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's 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 elders to Alexei. The responsibility and secrets of the Ark of Sirius were passed down to Alexei. He decided to seal the ark away to protect it, in the hope that the sirius tribe would not have to go to great lengths to continiue to hide the ark. To do so Alexi left Dogsville and sealed the ark at the cost of his own life.
Later when Yuliy was searching for the Ark of Sirius, he met Alexei at the Sirius Secret Chamber where the Ark of Sirius was sealed away together with Alexei's soul.
Alexi met Sachi, a Japanese human, and lived with her in Dogsville.
Mikhail looked up to his father as a child. He appears to miss him when he is gone and feels like he should take responsibilty, looking after Yuliy in his place.
Yuily can't remember Alexei as Alexi left when he was very young.
Alexei met Akasaka, a human, when he saved his children from a bear that was attacking them by killing it with his gun. Alexei is very grateful to him and they get to know each other. Despite the fact that Akasaka was sent by japanese military to find the ark, and sirius tribe rules stating he should be killed for that, they form a good freindship and Akasaka is the only person Alexei confindes where he sealed the ark in.