Toaru Majutsu no Index: New Testament - Volume 7, Chapter 4: Destruction of the Pre-Established Harmony. Total_Hero.

The protagonist of a war fantasy novel, Theo, competes with his rival and antagonist, Luke, for the position of Supreme Commander. Having been reincarnated into Luke’s role, he struggles to fulfill his duties as the villain. “It seems like principles as a soldier, the duties I should uphold, don’t matter to you at all.” Even if misunderstood by Theo, Luke silently remained faithful to his role as the antagonist. After completing all his duties, Luke successfully gave Theo his happy ending, cast aside his military uniform, and retired from the army. Now, Luke planned to create his own future story without interference from anyone. That was the plan, at least… “Why go this far? I’m no longer a soldier, and if you really think about it, we were never on good terms anyway.” To make matters worse, not only had he unexpectedly reunited with Theo, but now, Theo was persistently and seriously pressing him in a way he hadn’t imagined. “I want to know you better.” It was a completely incomprehensible statement, and it wasn’t even an appropriate answer to the question. The protagonist came to the antagonist and suddenly asked for help, simply because they wanted to know him better?

Crossing to One Piece world, he became Roger’s younger brother.

Twenty years ago, she was taken in as the Lin family’s young heiress through a twist of fate. But mistakes would eventually be corrected one day. When the real daughter of the family returned and her foster parents begged her to stay, she obliged. Unbeknownst to her, this was actually a scheme disguised as familial affection. They only kept her around to squeeze her dry of all her value!