Tsar Kaloyan Street in downtown Sofia was named after one of the prominent rulers of the Second Bulgarian Kingdom who was the only Bulgarian ruler to receive the Roman Catholic title of King.

Kaloyan was the third ruler of the Assen's dynasty who led the struggle for independence of Bulgaria from the Byzantine Empire at the end of the 12th century. Born in 1168, his name means the handsome and good. In 1187, he was sent to the Byzantine capital Constantinople where he spent two years as a hostage and a guarantee that the peace agreement between Bulgaria and the empire would be kept. In 1189, Kaloyan managed to escape from his captivity, but the experience turned the young man into a sworn enemy of the Byzantine people.

Kaloyan was pronounced tsar of Bulgaria in 1197, after the death of his brothers Assen and Petar who fell victim to a conspiracy. In his first years of ruling, the young tsar focused his efforts on establishing international recognition of the newly liberated Bulgarian state, using his skills as an excellent diplomat and statesman.

One of Kaloyan's main missions at the time was to form an anti-Byzantine coalition to fight his enemy and gain more territories for his state. He formed a surprising union with Ivanko, governor of the Plovdiv district, and also the murderer of his brother Assen. Kaloyan, however, overcame his hatred for the murderer and thought only about the glory of his state and his detest of the Byzantine Empire.

In 1199, Kaloyan received a letter from the Roman Pope Innocent III, in which the pope expressed his hope that the Bulgarians would join the Catholic Church. The tsar, however, was hoping to get official recognition of his title and his state from Constantinople. When it turned out that this would be a difficult task, Kaloyan began prolonged and complicated negotiations with Pope Innocent III. Kaloyan promised the union of the Bulgarian church with Rome, provided that the Pope would acknowledge his tsarist rank and the patriarchal dignity of the Bulgarian church.

In this way, Kaloyan continued the line of Bulgarian foreign policy which had been followed by his predecessors. The aim of this policy was to threaten the East (the Byzantine Empire) with the help of the West (Rome) and, in the political rivalry between the two big forces, to seek the realization of the interests of the Bulgarian policy. As a result of Kaloyan's correspondence with Rome, a Pope's legate brought a king's crown for him and a primate's title for the pastor of the Bulgarians. That, however, did not prevent Kaloyan from signing documents in his tsarist capacity and, in fact, not carrying the union into effect at all.

In 1204, during the Fourth Crusade, Roman forces conquered Constantinople and founded the so-called Latin Empire on the ruins of the Byzantine Empire. The crusaders showed an increasingly immoderate appetite for seizing Bulgaria and more than once arrogantly challenged its ruler.

This led to the battle of Odrin in 1205, where Kaloyan defeated the Roman army. The Emperor Baldwin was taken prisoner and in spite of Rome's threats to initiate a crusade against Kaloyan, Baldwin remained in Bulgaria's capital Turnovo until his death. There is not a unified opinion on exactly how Baldwin died. According to some historical chronicles, he was executed on account of the intrigues of the Bulgarian tsaritsa (queen) who was in love with him although the emperor did not return her affections. The neglected woman wanted revenge for her unrequited love and told her husband Kaloyan that Baldwin tried to seduce her. The Bulgarian ruler gave a command that the prisoner be executed.

After the Odrin battle, Kaloyan waged a merciless war on two fronts. On the one hand, he was fighting Romans, while on the other he was leading a war against the Greeks, annexing more land to his state. This prompted the Byzantine chroniclers to describe him as the most violent enemy of the Greek race.

Bulgarian tsar Kaloyan died in 1207, during the siege of Thessaloniki. He was killed in his sleep by a plotter named Manastre, who was the leader of a group of mercenaries in his army.