I grow stronger by making my wife happy.

Silver just wanted to grab a quick bite before heading back to work until he was shot dead. When he wakes up, he finds himself inside an apocalyptic novel… as the villain. A useless yet somehow talented one at that. Worse, he has a fiancée, a woman who sees him in the worst light ever possible thanks to the villain’s past actions. Now, to survive in a world overrun by beasts and chaos, Silver must do the impossible: win over the woman who hates him. Fortunately or unfortunately he gains a system. A strange one. A system that rewards him for making his wife happy. Sometimes, the rewards are small. Sometimes, they change everything. And sometimes… they affect both of them. — Description from novelcodex.org