Question: who is Constantine XI Palaiologos?

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Mesih Pasha: Mesih Pasha or Misac Pasha (died November 1501) was an Ottoman statesman of Byzantine Greek origin, being a nephew of the last Byzantine emperor, Constantine XI Palaiologos.  He served as Kapudan Pasha of the Ottoman Navy and was Grand Vizier of the Ottoman Empire in 1501.

Cleofa Malatesta: Cleofa Malatesta da Pesaro (also Cleofe, Cleopa or Cleopha) ("floruit" 1420 – died 1433) was an Italian noblewoman and the wife of Theodore II Palaiologos, Despot of the Morea, brother of Constantine XI, the last Byzantine emperor.  She was a daughter of Malatesta dei Sonetti, Count of Pesaro, and Isabella Gonzaga.  She married Theodore Palaiologos in Mystras on January 21, 1421, or sometime in 1422 in an arranged marriage that was part of an initiative of her uncle, Pope Martin V, to join Western (Roman Catholic) with Orthodox nobility, who in this way hoped to gain political alliances against the Ottoman Turks.

Fall of Constantinople: The Fall of Constantinople (Greek: Ἃλωσις τῆς Κωνσταντινουπόλεως , "Halōsis tēs Kōnstantinoupoleōs "; Turkish: ' Conquest of Istanbul) was the capture of the capital of the Byzantine Empire by an invading army of the Ottoman Empire on 29 May 1453.  The Ottomans were commanded by the then 21-year-old Ottoman Sultan Mehmed the Conqueror, who defeated an army commanded by Byzantine Emperor Constantine XI Palaiologos.  The conquest of Constantinople followed a 53-day siege that had begun on 6 April 1453.

Helena Dragaš: Helena Dragaš (Serbian: Јелена Драгаш, "Jelena Dragaš" , Greek: , "Elenē Dragasē"; c. 1372 – 23 March 1450) was the empress consort of Byzantine emperor Manuel II Palaiologos and mother of the last two emperors, John VIII Palaiologos and Constantine XI Palaiologos.  Later in life she became a nun.  She is venerated as a saint by the Eastern Orthodox Church under her monastic name, as Saint (Ὑπομονὴ ), translated in English as "Saint Patience".

Giovanni Giustiniani: Giovanni Giustiniani Longo (Greek: , "Iōánnēs Lóngos Ioustiniánēs"; Latin: "Ioannes Iustinianus Longus" ; 1418–June 1, 1453) was a Genoese captain, a member of one of the greatest families of the Republic of Genoa, a kinsman to the powerful house of Doria in Genoa, and protostrator of the Eastern Roman (Byzantine) Empire.  He led 700 professional soldiers, both Genovese and Greeks from the island of Chios, which at the time was part of the Republic of Genoa, to the defense of Constantinople against the Ottoman army of Sultan Mehmed II in 1453.  He personally financed, organized and led this expedition on his own initiative, and upon arriving was placed in command of the land defenses by Emperor Constantine XI Palaiologos of the Byzantine Empire.  Giustiniani was key in controlling the land forces and keeping the Greeks, Genoese and Venetians from quarrelling with each other, and instead kept focused on repairing the land walls after the Ottoman cannons had shot holes in them.  It was at least partly because of Giustiniani's charisma that the Byzantine forces were able to hold out so long against overwhelming odds.

Doukas (historian): Doukas or Dukas (  1400 after 1462) was a Byzantine historian who flourished under Constantine XI Palaiologos, the last Byzantine Emperor.  He is one of the most important sources for the last decades and eventual fall of the Byzantine Empire to the Ottomans.

Demetrios Palaiologos Kantakouzenos: Demetrios Palaiologos Kantakouzenos (flourished 1420-1453) was the "mesazon" (chief minister) of the Emperors John VIII Palaiologos and his brother, Constantine XI.  His colleague in the office as "mesazon" was Loukas Notaras.

George Sphrantzes: George Sphrantzes, also Phrantzes or Phrantza (Greek: Γεώργιος Σφραντζής or Φραντζής; 1401 – c. 1478) was a late Byzantine Greek historian and Imperial courtier.  He was an attendant to Emperor Manuel II Palaiologos, "protovestiarites" ("Lord of the Imperial Wardrobe") under John VIII Palaiologos, and a close confident to Constantine XI Palaiologos.  He was an eyewitness of the Fall of Constantinople in 1453, made a slave by the victorious Turks, but ransomed shortly afterwards.  Sphrantzes served the surviving members of the Palaiologian family for the next several years until taking monastic vows in 1472.  It was while a monk he wrote his history, which ends with the notice of Sultan Mehmed II's attempt to capture Naupaktos, which he dates to the summer of 1477; Sphrantzes is assumed to have died not long after that event.

Constantine XI Palaiologos: Constantine XI Dragases Palaiologos, Latinized as Palaeologus (Greek: Κωνσταντῖνος ΙΑ' Δραγάσης Παλαιολόγος , "Kōnstantinos XI Dragasēs Palaiologos", Serbian: Константин XI Драгаш Палеолог, "Konstantin XI Dragaš Paleolog" ; 8 February 1405 – 29 May 1453) was the last reigning Byzantine Emperor, reigning as a member of the Palaiologos dynasty from 1449 to his death in battle at the fall of Constantinople.  Following his death, he became a legendary figure in Greek folklore as the "Marble Emperor" who would awaken and recover the Empire and Constantinople from the Ottomans.  His death marked the end of the Roman Empire, which had continued in the East for 977 years after the fall of the Western Roman Empire.

Theophilos Palaiologos: Theophilos Palaiologos (Greek: Θεόφιλος Παλαιολόγος ; died 1453) was the cousin of emperor Constantine XI Palaiologos and a commander during the Fall of Constantinople.  Known as a grammarist, humanist, and mathematician, he died in the final battle alongside the emperor.

Supporting paragraphs: Helena Dragaš Constantine XI Palaiologos
Answer: emperor