Who is Ketura?
We know something about Abraham's two previous wives. Sarah was his cousin while Hagar was of Egyptian origin. About Ketura, however, the text is silent and gives no explanation[1], leaving it to the commentators to express their opinion.
Rabbi Judah says in the Midrash (Bereshit Rabba 61, 4): "She is Hagar". This is the opinion of the Zohar as well (133b) and of Targum Jonathan. As we saw above, Rashi agrees with them.[2] However, the commentators who explained the text in its literal sense (peshat) did not accept this. Rashbam states in short: "According to the peshat, this is not Hagar", without providing any explanation of his opposition. The difficulty might be as formulated by Rabbi Nehemia in the Midrash (ibid.): "Behold, the text reads va-yosef, literally 'he added'. [Gen. 25:1 reads: Abraham took another wife, whose name was Keturah. The Hebrew actually says "He added on another wife".] If Hagar were identical with Ketura and Hagar was already his wife, there would have been no need to say about her that 'he added' her to his wives.
Ibn Ezra raises another difficulty: "Ketura is not Hagar, because it is written: "But to Abraham's sons by concubines" (25:6). In other words, the text is not talking about one concubine but about two, at the least. So we must understand that Ketura was added to Hagar. For Rashi this does not constitute a problem, because in verse 6 he wrote: " 'and for the sons of the concubine' [pilagshim is written defectively, without the second yod]- defective spelling, as there was only one concubine, who is Hagar who is Ketura".[3] But in the Massoretic Text of the Bible as we have it today the word is actually written pilagshim, with plene spelling which indicates the plural.[4]
If Ketura is not Hagar, as many of the commentators maintain, we would like to offer a possible identification. G-d's last promise to Abraham in His first revelation was: "And all the families of the earth shall bless themselves by you" (12:3), and apparently this was the most important promise. What is its significance? Does venivrekhu really mean 'bless', as understood by some commentators (Rashi, Ibn Ezra, Radak)? According to Rashbam (on the parallel verse, 28:14) the meaning that comes to mind is 'to graft' a branch, and the connotation is 'to bless by combining', in other words the families of the earth will intermingle with your family.[5]
If we accept this second meaning, we find it to be the exact opposite of he commandment of G-d to Abraham to "Go forth from your native land and from your father's house" (12:1). That commandment pointed Abraham in the direction of separation and withdrawal from other men, while this blessing points to renewed connection. Hence the entire experience of severance from his society and from his father's house had only one purpose - to make it possible for Abraham to crystallize and shape his personality.[6]
If indeed Abraham was destined to once again maintain contact with all the peoples of the world, we can now consider the possibility that Abraham's three wives - Sarah, Hagar and Ketura - represent his connection with the three families of the earth, for it was through the three sons of Noah-- Shem, Ham, and Japheth-- that humanity was again established after the flood.
And we indeed found the following words in the midrash anthology Yalkut Shimoni for Job (903): "Abraham married three women: Sarah, the daughter of Shem; Ketura, the daughter of Japheth; Hagar, the daughter of Ham". And in fact he married them in the order in which the "fathers" appear in the Bible (6:9) - first the daughter of Shem, then the daughter of Ham, and finally the daughter of Japheth.
By marrying these three women, the blessing that G-d bestowed upon Abraham, that "all the families of the earth shall bless themselves by you", was fulfilled, and similarly when he produced offspring from these three women, the blessing that he would be "the father of a multitude of nations" was also fulfilled.
http://www.biu.ac.il/JH/Parasha/eng/chaye/sha.html