Sima Yi (司馬懿), style name Zhongda (仲達) is a Three Kingdoms historical figure who was known to originally be a vassal to the Wei kingdom, helping his Lord Cao Cao in his ambitions as a very brilliant tactian. He then serves Cao Cao's son, Cao Pi. However, it was often stated by Cao Cao that Sima Yi never had much intention of serving below someone, a fact that came to fruition during the aftermath of Zhuge Liang's Five Northern Campaigns against Wei when Sima Yi took control of the falling Wei empire, and went on to establish the foundation of the upcoming Jin dynasty.
He is often considered a main antagonist in the later parts of the Romance of the Three Kingdoms adaptations. Historically, he and Zhuge Liang didn't exactly have the well-established rivalry seen in most fictional accounts. His wife is Zhang Chunhua, while his two historical children are Sima Shi and Sima Zhao.
Sima Yi started serving the Wei forces as a simple vassal, albeit a genius one that puts his allies to shame. He works his way up the ranks, due to his repeated successes and strategies against both of Cao Cao's main rivals, the Shu forces and the Wu forces. After Cao Cao's death, he is appointed as supreme warlord by emperor Cao Pi.
Meanwhile in Shu, the legendary and unparalleled military genius, Zhuge Liang, starts to feel threatened by Sima Yi and begins an expedition into Wei.
After many battles and years, Zhuge Liang and Sima Yi's rivalry reaches a stand off at the Battle of the Wu Zhang Plains. During this legendary battle. Zhuge Liang falls gravely ill, and decides to end the battle quickly. Sima Yi, knowing that without Zhuge Liang the Shu forces are nothing, plans for a long, drawn out battle, hoping for Zhuge Liang's death.
After Zhuge Liang dies, Sima Yi wins the battle. Although they were rivals, Sima Yi has great respect for Zhuge Liang, and orders a memorial in honor of his fallen rival. A few years later, Shu is conquered, and Wei looks as if it will unify the entire land. However, Sima Yi, along with his sons, see this as an opportunity. Although the Cao family is the ruling family, the Sima family are actually the ones that have garnished the most power, including the army. The Sima family successfully overthrows the Wei Empire, and establishes the Jin Empire. After which they set their sights on the Kingdom of Wu. Seeing that Jin rule is inevitable, Wu surrenders. China is unified under the Jin Empire.