In the context of Genesis 48
The word "Angel" in Genesis 48:16 (KJV) is the Hebrew word הַמַּלְאָךְ֙ (ham-mal-’āḵ). This particular form of the wordHebrew root מַלְאָךְ (mal-’aḵ) is found 23 times in the KJV, but only in this instance does it have a capital letter.
I'm not an expert in the Hebrew language, but I'm pretty sure there isn't anything special about the word הַמַּלְאָךְ֙ (ham-mal-’āḵ) that would justify a capital. It is likely just a preference of the KJV translators because Jacob wrestled with what he thought was a man, but about whom he declared:
...I have seen God face to face, and my life is preserved.
-- Genesis 32:30 (KJV)
In other contexts
Other instances of הַמַּלְאָךְ֙ (ham-mal-’āḵ)
2 Samuel 11 -- four times translated as "messenger" in reference to a human being (vv. 19, 22, 23, 25).
2 Kings 6 -- twice translated as "messenger" in reference to a human being (vv. 32, 33).
2 Kings 9:18 -- translated as "messenger" in reference to a man on horseback.
2 Kings 10:8 -- translated as "messenger", but there is nothing to suggest he is anything other than human.
1 Chronicles 21:20 -- translated as "angel" in reference to an angelic being God sent to destroy Jerusalem.
Ecclesiastes 5:6 -- translated as "angel". The reference here isn't clear, but the sense of it is: "the presence of God".
Zechariah 1, 2, 4, 5, 6 -- 12 instances, all translated as "angel" in reference to angelic beings with whom Zechariah converses. The references are very reminiscent of John's encounters recorded in Revelation.