Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And We brought them in) We shall enter them in the Hereafter (unto Our mercy) to Our Paradise. (Lo! they are among the righteous) they were messengers, except for Dhu'l-Kifl who was not a messenger but a righteous man.
And We admitted them into Our mercy, through [granting them] prophethood. Indeed they were among the worthy, of it. Dhū’l-Kifl was so called because he undertook (takaffala) to fast every day and stay up every night in prayer, and to pass judgement between people without succumbing to anger, and fulfilled this [undertaking]. It is also said, however, that he was not a prophet.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
For explanation, see Surah Maryam (XIX): E. N. 33.
Zul-Kifl is not the name but the title of a righteous man, which literally means 'a man of luck'. Here it does not refer to worldly prosperity but to his high character and ranks in the Hereafter. He has also been mentioned by this title in XXXVIII: 48. There are different opinions about his identity and nationality: some have regarded him as Zacharias (but this is not correct because Zacharias has been mentioned separately in v. 89); others say he was Elias, or Joshua, son of Nun, or Elisha, but this again is incorrect, because in Surah Sad (XXXVIII: '49) Elisha and Zul-Kifl have been mentioned as separate personalities; some others say that he was Prophet Job's son, named Bishr, who succeeded him as Prophet.
Allamah Alusi says, "The Jews claim that he was Ezekiel who was appointed to Prophethood during the captivity (597 BC) of the Israelites and he performed his mission in a habitation by the side of the Chebar canal."
These conflicting opinions indeed confirm nothing. The modern commentators, however, are inclined to believe that he was Ezekiel, though there is no convincing argument about it. This opinion is sound because his description in this verse "that he was a patient and righteous man and was blessed by God" is fully confirmed by the Book of Ezekiel. He was one of those people who had been taken prisoner by Nebuchadnezzer at the downfall of Jerusalem, who settled the Israeli exiles at Tel-abib by the river Chebar in Iraq. Here, in 594 BC, Ezekiel was raised to Prophethood when he was hardly 30, and he continued preaching the message of God to the exiled Israelites as well as to the iniquitous people and rulers of Jerusalem for full 22 years. In the 9th year of his mission, his wife whom he called "the desire of his eyes" died, but when the people came to mourn her death, he warned them of the wrath of God and the impending disaster. (Chapter 24: I5-21). The Book of the Prophet Ezekiel in the Bible is one of those scriptures which appear to be genuine and divinely' inspired.