Piotr Chekov

From Memory Alpha, the free Star Trek reference.

Piotr Chekov was the fictional brother of Pavel Chekov, created by the manipulative Beta XII-A entity.

The entity had created the memories of Chekov's phantom brother and his untimely death at the hands of Klingons on Archanis IV in an effort to stir up anger and hatred in Chekov towards the survivors of a wrecked Klingon battle cruiser, so that it could feed off of the young Russian's emotions of hate and violence. (TOS: "Day of the Dove")

As Sulu noted that Pavel did not have a brother, it is unknown what, if any, basis in reality the entity used in its portrayal of the fictional Piotr.