Data's older brother and a malevolent android.
Lore is an android built by Noonien Soong before Data and physically identical to him, but capable of emotion and lacking his brother's conscience. The colonists of Omicron Theta grew uneasy with him and he was disassembled. In the first-season episode Datalore the Enterprise crew finds and reactivates him, and he soon tries to deliver the ship to the Crystalline Entity before Wesley Crusher beams him into space.
In Brothers, Lore answers the same summons from Soong that brings Data home, steals the emotion chip intended for Data and mortally wounds their creator. In the two-part Descent he takes control of a group of Borg cut off from the Collective and uses the stolen chip to manipulate Data's emotions and win him over. Data breaks free of his influence and deactivates him.
In the third season of Picard, Lore's personality resurfaces inside the synthetic body that also holds Data, and the two struggle for control until Data prevails.