Both speak with very rough voices and have shady backgrounds, both rode in a gang and were betrayed by their gang's leader after they had gone insane.
Both Marston and Klebitz are called Johnny Boy by other characters.
Both are possibly blind in one eye.
Both are canonically killed on screen.
Both were killed by a person that they had an uneasy working relationship with, in Marston's case it was Edgar Ross, while in Klebitz's case, it was Trevor Philips.
When both Marston and Klebitz were killed, their bodies were cradled by the women they loved. In Marston's case it was his wife, Abigail, while in Klebitz's case it was Ashley Butler. Both Abigail and Ashley would later die as well (Abigail dying from an unknown cause, and Ashley dying of a drug overdose).
Both have a skeptical and hating view of how the US Government operates, yet, they are forced to work for a government agent/politician in order to save themselves and the ones they care about (In Marston's case it was himself and his family, and in Klebitz's case it was himself and his club).
Both speak a little Spanish through the game.
Women of both were "every man's wife" or the gang's whore at some point of their lives.
Both were involved in kidnapping at least once. In Marston's case it was bounty hunting, while in Johnny's case it was kidnappingNiko'scousin.
Johnny is nicknamed Cowboy by Trevor, which might emphasize these similarities.
If spawned during gameplay using trainers, he will have no legs and no lower arms, making him look like floating mid-air. That's because the rest of his body never appears, but only the head and a bit of his chest. This is due to Rockstar making only part of him for the photo when choosing parents.