Snowstar

Mark is a genius military weapon designer who was kidnapped by a mafia gang after refusing to work for them.
They forced him to design unique weapons for five years, until one day he successfully escaped only to get shot by his trusted comrades.
*Ding! You awakened a weapon system.
When he opened his eyes, he was in a different body and in a different world.
"You want to kill a wild boar without being detected? Forget about bow and arrow. Here, I present you this sniper rifle."
"You want to kill a horde of goblins from the sky? Forget about those flying carpets and brooms. Here, I present you the fighter jet." 
"You want to kill a dragon? Forget about taking the help of the S-class adventure team. Here, I present you a nuclear bomb."
Join Mark Spencer/ Lu Zhen as he brings the whole world to its knees with the help of his weapon system.
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Note: Author isn't born in an English-speaking nation and has no editor either. So, please be understanding if there are any grammatical errors.
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Author's current works: War Online, My Soul card is a Reaper
Author's other notable works:  The Last Slytherin, The Sharingan Hyuga
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You can also support the Author through Patreon

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Weapon seller in the world of magic

Originally, Mark planned to reveal his real identity and further tell her all things about Allen to prove his point in order to gain her trust so that he could get access to all her memories and find a way to his home planet.

But, after hearing her story, he realized that it is useless and he changed his plans swiftly. He doesn't see any reason to access her memories now. Without his younger brother's presence at either Ixitus Emperor or his home planet, he has no motivation to visit those places.

Yes, there's an afterthought of taking his revenge against his former superior, the friends who betrayed him, and the mafia gang group that turned his life upside down, but still, they aren't as important as his younger member. Finding him takes priority and now Mark has no idea how to accomplish the task.

As a result, he took the approach of being a close friend to Mark who heard about her, that's all.

The Emperor summons Mark