Ptolemy gives Regulus the nature of Mars and Jupiter, and the tradition that follows reads it as the most kingly of the fixed stars: power, high office, ambition realised, martial honour, with the standing caveat that its gifts are lost through pride or revenge and end in a sudden fall.
The two components divide cleanly. The Mars half supplies the courage and the military honour, which is the classic profile of a royal significator in a world where kingship meant commanding armies. The Jupiter half supplies the rulership itself, the office and the elevation. The caveat about the fall is not Ptolemy's and belongs to the later tradition, principally Robson.
Regulus is one of Agrippa's Behenian fifteen, paired with garnet and with mugwort, a specific talismanic correspondence with a named Renaissance source, and a rare case where a star, a stone and a plant are all documented as a set by one author. It is also counted among the four Royal Stars, as the Watcher of the North, in the scheme discussed under Aldebaran and best treated as unsourced.
There is a genuine ancient layer beneath all of this. Regulus is the king star in Babylonian celestial omen literature, where its significance is for the state and the monarch rather than for individuals, and that is the oldest attested royal association it has. The star's kingly character is therefore about as well established as any in this material (it appears independently in Mesopotamian, Greek, Persian and Indian traditions), even though the specific delineations printed today are modern.