Toaru Majutsu no Index - Volume 3, Chapter 3: Railgun. Level5.

In the era of interstellar warfare, every melee mech will be paired with a long-range sniper mech to cover the battle.
The sniper mecha master Yan Xueshan (beta) and the melee mecha master Arthur (alpha) are just such a pair. The two have been together for six years.
The upcoming Armageddon is a near-death battle.
the day before.
Yan Xueshan: “Remember to write a suicide note in advance.”
Arthur said frustratedly: “I have only one regret. If I die this time, I will be a virgin until I die.”
Yan Xueshan snorted coldly: “…no promise.”
Arthur asked pitifully, “Master, can you fulfill my dying wish??”
Yan Xueshan: “?”
How absurd.
But Yan Xueshan thought about it, this is his comrade-in-arms, whose life and death intersect, chastity counts as a p- OK, I agreed! !
However, after the Armageddon, none of them died.
Not only did they become war heroes, Arthur also turned into a federal prince.
Arthur took out the marriage application, just one signature, and said honestly and faithfully: “Master, didn’t you say that you will go back to get married and retire after this battle? You help me fulfill my wish, and I will help you fulfill my wish too!”
Yan Xueshan: … something seems to be wrong?

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Search keywords: Protagonist: Yan Xueshan, Arthur ┃ Supporting role: ┃ Others:

- Description from novelbuddy

In a tyrannical world, what is evil? What is benevolence? All living creatures are brutal. Passing through the heaven road to find one’s purpose in life. Struggles to be stronger is not an easy path. The whole journey thistles and thorns, one blade chops open a path, I alone will be supreme!

- Description from Novelupdates

This world is sick; both Immortals and mortals suffer from a stubborn ailment known as Longevity.Under the influence of Immortality, various Cultivation Techniques evolve rapidly in grotesque and inexplicable directions.[Fetal Period: A baby is born with shriveled, powerless limbs.][Childhood: After 20 years, the Internal Organs completely decay.][Weak Crown Period: At 50 years, the flesh dries and withers.][Thirty Years Old Stage: After 100 years, the blood runs dry and gives birth to mutations.]I am an Immortal Seeker, and also one seeking survival.