Chesed is mercy, the fourth sephirah, the unmeasured outpouring that Geburah below it has to restrain. Its planet is Jupiter, and Jupiter in Hebrew is Tzedek, righteousness. Tzadkiel means the righteousness of God. Name, planet and sphere all sit on the same root, which is tidier than most of this subject manages.
The story most often attached to him is the binding of Isaac. Some Jewish traditions name Zadkiel as the angel who called from heaven and stopped Abraham's knife. Others name Michael. Others name Tadhiel, a name that appears almost nowhere else. Genesis 22 names nobody at all: the text has the angel of the LORD, unidentified, and says no more.
That sequence is worth watching, because it is how a great deal of angelology was produced. An unnamed figure in scripture attracts a name, different communities supply different names, each name accumulates a small body of associated lore, and later compilations list the results side by side as though they were separate angels with separate offices.
A second layer of accumulation is much more recent. Twentieth-century angel devotion, working from theosophical sources and then from the New Age literature that followed them, has given Zadkiel a fixed association with forgiveness, mercy and a violet flame. None of that is in Agrippa, in the Golden Dawn tables or in the Jewish material. It is a real tradition, and it is a modern one, and the two should not be quoted as one.