The Punisher isn’t the most social guy. Not the best with people, and doesn’t have much luck with sidekicks.
For Frank Castle having a partner, or sidekick just slows him down. He isn’t as fast, isn’t as quiet, and just isn’t at his most lethal. The Punisher isn’t looking to be part of a group or to fight alongside anyone.
In past story lines, such as Civil War, the Punisher has fought with the Avengers, the Thunderbolts, and other super powered people while fighting for a bigger goal. Saving the world or something like that, but Castle has a a nasty habit of fighting for a while, then fighting some more, then fighting his allies, and then going back to curb-stomping thugs in NYC. Sidekicks or partners aren’t really his thing.
ThePunisher has worked with people for short periods of time, but don’t stick around very long. Martin Soap. Microchip. Other heroes come and go, he fights along side but never too attached. As the Punisher, he’s lost his attachment to people, knows better than to risk his sanity (or lack thereof) again. We’ve talked about the Punisher’s problems before, the main one being any attachment to the world, besides his family had nothing, now he has nothing but killing. The Punisher is too self-aware to put himself in a position to lose it all again, even he knows his limits.
In the comic book universe sidekicks are usually found or created, they suffer similar fates to their heroic mentor i.e. family is murdered, irradiated, lost in space, out for vengeance, whatever the typical stuff that makes heroes. A healthy dose of psychological scarring and post-traumatic stress.
Maybe Frank Castle could have a sidekick one day, but for now he works alone and it’ll probably stay that way. As much as we love The Punisher, it would seem he’s destined to die alone, no sidekick, no legacy to be carried on, and no one to bury him.