It's Really Hard for Me to Save Future Humans

"I used to have an afterlife attack when I suffered from Chuunibyou disease before I died. I didn't expect that I would survive one day. And I am now a mysterious agent in ancient human history who was resurrected in modern times to fight to protect humanity - yes, my current combat effectiveness is really equivalent to that of a top agent. And I discovered this... System after my resurrection? It allows me to contact you. Once you convince the people of your era that you do have a certain identity, then I can obtain the corresponding abilities." "??? If this didn't appear directly in my mind, I would have thought it was a joke." "Then, I'll leave it to you! It's my duty to rebuild the glory of mankind as I once was!" "I... Try my best." Host, please select the first ability... Note: The male protagonist is a modern human being, and the future male protagonist can be regarded as the second male protagonist. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.