Sergei Valishnikov was a human that worked as a drug lord.
When Deadpool was hired to kill gangsters and criminals across the world, Sergei was one of the mercenary's targets. During and incursion at the drug facility, Sergei runs away and hides inside his panic room while Deadpool fights with Sergei's subordinates. Deadpool is annoyed by Sergei's cowardice and urges him to come out and fight due to it being his anniversary with his girlfriend Vanessa Carlysle, but Deadpool is ultimately forced to escape with help from Dopinder when the rest of Sergei's gang members arrive.
Wanting to get revenge on Deadpool, Sergei and his men track him down to his girlfriend Vanessa's apartment. Deadpool succeeds in killing all of the henchmen attacking him, and Sergei himself emerges to shoot the mercenary. In response, Deadpool throws a cream cheese spreader at him, although this causes the angle of his gun to shift, resulting in Vanessa being shot instead. Totally devastated, Deadpool chases Sergei down as he is trying to escape and corners him in the middle of the street.
However, rather than kill Sergei immediately, Deadpool hugs him instead, much to his confusion. Deadpool then jumps in front of a moving truck while still holding onto Sergei, killing him. Despite his death, Sergei's actions leave a massive impact Deadpool. Vanessa's death causes Deadpool to stumble into a depression, trying to fruitlessly kill himself to no avail due to his regenerative abilities, and leading to him being recruited by Colossus to join the X-Men.
Deadpool eventually has Cable's time travel watch repaired, and uses it go back in time to the exact moment following Deadpool's dispatching of Sergei's underlings after they invade at his and Vanessa's apartment. Sergei appears with his gun pulled out, but this time, Deadpool threw the cream cheese spreader at Sergei's head, killing the drug lord instantly, and successfully saving Vanessa from death.