The Hebrew of the verse is as follows:
כה־אמר יהוה מלך־ישראל וגאלו יהוה צבאות אני ראשון ואני אחרון ומבלעדי אין אלהים׃
The word in question is וגאלו (w'galow) and is composed of the root verb גאל (gaal), meaning to redeem, with a beginning prefixed vav (ו), which is the standard conjunction meaning 'and,' but which functions various ways, including even 'but;' here, it functions appositionally (elaborating upon or defining the previously mentioned noun). Then the suffixed vav (ו) signifies 'of him.' So the one word means 'and [the] redeeming [i.e. redeemer] of him,' that is, 'and his redeemer,' or 'even his redeemer.' The object Israel is the referent of the possessive 'of him,' meaning The "Redemer" of Israel is a description of "the Lord" and "King" mentioned before.
Sometimes the literal translation can obscure the meaning by giving false impressions. We might translate this better to understand what the sense is, namely:
Thus says the Lord, King of Israel: his redeemer, the Lord of hosts: "I am the first and the last; apart from me there is no God."
As a Christian who believes that Jesus is the Lord God (YHVH) in the flesh, and the Redeemer, this passage is not attempting to identify a certain Redeemer as distinct from the Lord (the Lord and his friend the Lord of hosts), but that the Lord (of Hosts) is the Redeemer of Israel.