Lionel grew up in New York. He knew how it worked since he was five. His parents were nothing to write home about, and neither was his ex-wife.
The only thing he was truly proud of is his son, Lee who has been raising on his own since his ex took off.
He became a cop, because it was the good thing to do. He was stationed at 51st precinct and ended up working with James Stills and Louis Azarello. Soon they pulled him further into corruption, until he was completely snared and working for HR.
He found his redemption when he got the the first and only drop on John Reese. Stills ordered Fusco to kill and dump John’s body in Oyster Bay. During the ride out, John facilitate an escape and let Lionel live, realizing he still was a good cop.
He made Lionel into an Asset, pulling him in to help with the numbers.
John had Lionel moved to the 8th Precinct to work with Carter.
John had Lionel go deeper into HR, to help bring down the organization. It was then he realized he was finally doing something good. He liked working with two men, and the dog was cute.
In time Lionel discovered that Carter was also working for them, when the two were trying to help him stop an HR hit. Only to watch John hunt down the bitch that had stolen Harold.
If that wasn’t enough to prove there was something more going on … after seeing John jump across a four lane highway to get to another building to save Finch, he was sold on the Vampire thing.
He was there for Carter when she discovered about Zoe and later Turned.
She in turn was there for him, when he discovered about Donna.