The boy was adopted by a good-hearted, wealthy businessman as his sixth child, but continued his behavioral problems, getting into fights in school and doing criminal acts, but just when he was starting to improve due to finally feeling loved by his father, the businessman and his wife were killed, causing him to go insane, but there was a speculation that the boy murdered him when he became paranoid of the fact that his adopted father wouldn't love him anymore. McCall makes a speech at the middle of the film after Teddy asks if he thinks the knows him, and McCall starts talking about a young boy who was abused and was having a horrible life, having never been loved, who trouble found early. Teddy Rensen is a very enigmatic hitman, but there is a shade of backstory given.

Nikolai has an habit to play with his prey before killing them, but that habit leads him to underestimate his opponents, specially Robert McCall, who is two steps ahead of him, both strategically and military, costing his boss Vladimir Pushkin money, resources, influence and clients due to mismanaging the situation poorly and thus, his own life at the hands of McCall. Having a sociopathic mindset, Nikolai considered himself to be free from emotions, having no remorse or regret of the things he has done in his life, However as Robert McCall stated using a story of an orphan who killed his adoptive parents as a allegory, Nikolai is a person who use violence to cover what he really is: 'a scared little kid'. And thanks to his military background, he is an expert at hand and gun combat. Teddy is a very violent and sociopathic person who takes pleasure in killing and torturing, evidenced when he beats a man into bloody pulp over 'nothing' and calmly strangles a girl to death for not telling him where McCall is and also using bystander to lure McCall.

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