The Author is an Extra, The MC is a Reincarnator - Chapter 6 Problematic Reincarnator

After time-traveling, Su Yunjin thought she had married a lame brutish fellow. As it turned out, not only could the hunter husband do business and hold office, but his leg wasn't lame either.“Wife, take a seat, I'll make the money.”“And what about me?” Su Yunjin raised an eyebrow.The man chuckled lowly and pulled out two cute little buns next to him. On each side, the adorable buns clung to Su Yunjin's thighs, “Mommy, we want a little sister!”

This legend tells the tale of the protagonist, Kuroki summoned to protect the demon king from the hero.

Kuroki, unbeknownst to him, suddenly came to another world. He was summoned by the demon king, Modes. The territory which the demon king overrules was in the midst of being destroyed because of the hero. He had no choice but to bow before Kuroki.

“Please save us, Savior Dono!”

Kuroki nodded, deciding to save the demon king due to an unexpected turn of event.

Being given a chance and reincarnated is nice and all, yet why do I have to become that male friend who just watches the protagonist flirting with his harem?---World: Kaguya-sama: Love Is War - Hyouka ----

He lived from BC to the Middle Ages, from the Renaissance to the Industrial Revolution, and from Cyberpunk to the Interstellar Age. He has been a king and a prophet, a knight and a nobleman, a versatile artist, a genius inventor…
This is a four-thousand-year human epic——
“To us four thousand years from now:
“Is the world at peace? Is famine extinct? Can people touch the stars? How long is the human lifespan? How far is the boundary of the earth? Human…do you remember us?”
——Excavated from Earth Age Tenth Century BC, Solomon Prayer Book.

- Description from novelbuddy