History of the star: A star located near the left hind quarter of the Centaur Centaurus.

Agena is said to come from a and gena, "the knee". The Arabic name for Agena is Hadar, "Ground". "The settled land" was another meaning given. This star and alpha (Toliman, also called Bungula) are the Southern Pointers, i. e. towards the Southern Cross (Crux), often regarded as the Cynosure of the southern hemisphere. Arabic Wazn, "Weight", seem to have been applied to alpha and beta of this constellation, and to stars in Argo, Columba, and Canis Major, probably on account of their proximity to the horizon.

[Star Names, Their Lore and Meaning, Richard Hinchley Allen, 1889].  

The astrological influences of the constellation given by Manilius:

"The Centaur, from his own nature assigns qualities to his progeny. Such a one will either urge on asses with the goad and yoke together quadrupeds of mixed stock or will ride aloft in a chariot; else he will saddle horses with a fighter or drive them into the fight ("he will load horses with arms or drive them into arms", that is yoked to war-chariots).  

Another knows how to apply the arts of healing to the limbs of animals [translator's note; these endowments reflect the identification of Centaurus as Chiron] and to relieve the dumb creatures of the disorders they cannot describe for his hearing.

His is indeed a calling of skill, not to wait for the cries of pain, but recognize betimes a sick body not yet conscious of its sickness". [Astronomica, Manilius, 1st century A.D, book 5, p.329]. 

The general astrological influences of the star: It gives position, friendship, refinement, morality, health and honor. (Robson*).

Gossip or scandal, if with Venus and Mars, or Venus and Saturn. Success and happiness, if conjunct a benefic. (Fixed Stars and Their Interpretation, Elsbeth Ebertin, 1923)

A significator of success. (Larousse Encyclopedia of Astrology).

With Sun: Mental activity, rashness, success, many friendships. (Robson*).

With Moon: Sarcasm and bitter speech, strong passions. (Robson*).

With Mercury: Great mental ability, sarcasm, material gain, speaking, writing and championing the masses. (Robson*).

With Venus: Poetical, strong passions, rash friendships, with women. (Robson*).

Pronounced sensuality. (Fixed Stars and Their Interpretation, Elsbeth Ebertin, 1923)

With Mars: Legal authority, honor as speaker or writer, great mental and physical powers. (Robson*).

With Jupiter: Intellectual success, legal and Church authority, professional honor. (Robson*).

With Saturn: Thoughtful, shrewd, association with medical men, healing powers, abrupt manner, occultism, domestic disharmony through jealousy. (Robson*).

With Uranus: Sarcasm, public work but eventual disfavor of superiors, caustic speaker arid writer, violent, coarse, unrefined, strong passions, domestic disharmony, loss of expected wealth, peculiar death. (Robson*).

With Neptune: Sharp, rash, headstrong, original, organizer, poor executive ability, loss through law and speculation, obstacles to success, many false friends and enemies, liable to accidents, death by colds or fevers. (Robson*).

References

*(Fixed Stars and Constellations in Astrology , Vivian E. Robson, 1923)