What does the guardian star of the twelve constellations represent?

Do you mean the ruler of the twelve constellations?

Mars vs Aries (masculine; Violence)

The moon is against cancer (irrational; Mystery; Affectionate)

Sun vs Leo (enthusiasm; Wise; Vigor)

Mercury vs Virgo and Gemini (wisdom; Agile; Variable)

Venus vs Libra and Taurus (love; United States)

Pluto vs Scorpio (extinction; Rehabilitation)

Jupiter vs Sagittarius (lucky; Wealth)

Saturn vs Capricorn (limit; Power; Selfish)

Uranus to Aquarius (rationality; Create)

Neptune vs Pisces (change; Trigger inspiration)

PS: Before the discovery of trigeminal nerve, Scorpio was guarded by Mars, Aquarius by Saturn and Pisces by Jupiter.