From Yen Press:
Momoko is the only child of the Kuzuryuu clan, a powerful martial arts family with inhuman strength. But strong as she is, her father fears a woman will never be able to master the skills of the family. Unless Momoko marries and bears a capable heir, the Kuzuryuu line is doomed to weaken and die off! With this mission in mind, Momoko is sent to the rival Inuzuka clan, where she is to marry the son of their mighty leader. But Koushi Inuzuka abandoned the martial arts long ago. Now committed to his legal studies, the last thing Koushi has on his mind is marriage — especially not to the world’s strongest bride!
Until the day that Momoko Kuzuryu, daughter of another great house, drops into his life and demands that he, the last scion of the strongest family of bushido martial arts, sire a baby with her.
From that moment on, Koushi is forced to think and act faster than ever before as ever-stronger opponents appear to challenge him. The race to stay alive hinges on unlocking memories from his suppressed past and, possibly, unleashing his forgotten physical courage. And somehow, at the center of all those memories, stands Momoko.
How can a wimpy bookworm survive the attention of skilled assassins for long? Then again, his opponents are discovering that some forms of strength have nothing at all to do with muscles or techniques...