Not much is known about Reynolds before his days in the zombie world.
Mayor Story Arcs
Reynolds seems to have a keen intellect and was attempting to find a cure for the zombie virus by using the decapitated head of the Wasp.In the 40 years that followed, Reynolds developed a close friendship with the Wasp, one that didn't end there as, after a battle in which Wasp is injured, he reveals that he in fact loves her; a feeling she seems to reciprocate as they share a kiss that shocks all onlookers, zombie and human alike. This kiss and its failure to infect Reynolds seemingly indicates that the zombie virus can't be passed along through saliva (pure speculation).
In addition to his relationship with Wasp, over these 40 years Reynolds assited Forge (or maybe Forge assisted him; who can say) in inventing the force field the survivors use to repel the returning Zombie assault. He had also tinkered with Wasp's design so much that he had spare bodies to use for when the still "living" head of Hawkeye is brought to camp to receive the same treatment.
Perhaps most importantly, Reynolds and Forge had performed the experiment upon Black Panther's son's corpse, without Panther's knowledge, wherein they had transplanted the zombie Colonel America's brain into the corpse and managed to turn the reanimated body into a fighting machine for their cause. This angers Black Panther to no end and he'd probably have killed them both if not for being restrained. He doesn't forgive them for what they did to his son's body but he accepts it as a necessary thing as he fights alongside his son's reanimated body (though not with his son's mind in it) to repel the zombie assault.
After a battle in which Wasp is seemingly killed, Reynolds, despondent, allows himself to be eaten by the zombie Hulk; knowing that sating Hulk's hunker will calm him enough to revert him to Banner and end his rampage that had killed many. Reynolds' sacrifice was panned out, allowing for Banner to voluntarily be killed before he could become Hulk again; saving the day.