Head Researcher

"Head Researcher" is an unnamed person who lived and died centuries before the main story begins. He is sometimes also referred to as "Idiot Inventor," "Stupid Chief Engineer," "The Scientist," or some other names along those lines. He was born in modern Japan and sent to the distant-past Fantasy World as a Reincarnator to fight the Demon King of that time. While we never get to meet him when he was alive, we do see glimpses of his life from his research diaries.

(Source: KonoSuba Wikia)