[Single heroine + post-apocalyptic sci-fi + happy ending + lots of scenes with the leads fighting side by side + both leads are strong + devoted to each other]
In 2044 on Blue Star, the Bear Country had already lost half its territory in the war against the Eagle Country, and its capital was on the verge of being captured. The Bear Country's leader ordered the launch of every nuclear missile, striking nearly every landmass except the Dragon Country and a few countries in the Continent of Feizhou. To wipe out the Eagle Country's invading allied ground forces, the Bear Country even launched a nuclear attack on its own territory.
The Bear Country had produced so many nuclear missiles during the war that the Eagle Country's missile defense system could intercept only some of them. The Bear Country's plan had nearly succeeded: almost all lands apart from the Dragon Country and Feizhou were completely destroyed, and the Eagle Country and its allies were almost entirely wiped out. However, the Eagle Country still had several small nuclear test sites hidden in a country in Feizhou and on a small island. It also still had several huge fleets out on the vast ocean.
Amid the tense atmosphere, the remaining senior officers of the Eagle Country's military held an emergency secret meeting. They knew they were already at the end of the line, but were still determined to use their last resort against the powerful Dragon Country. After careful consideration, they decided to deploy their remaining nuclear weapons in a devastating strike against the Dragon Country's military bases, major power plants, and other critical infrastructure.
Wang Yu, a key researcher at a controlled nuclear fusion experimental facility, awoke amid the explosion to discover that he was the only survivor in the entire facility.
Wang Yu also discovered that his body had undergone some changes due to the radiation...
I haven't seen the mecha. I came to read your book because of the mecha. You told me that there is nothing I want to read.