This is a very early example of Ali's Baba, a strip which appeared in D. C. Thomson's Sparky from January 1970 up until the comic's end in July 1977; it was drawn by Malcolm "Mal" Judge, better known for D. C. stablemates such as The Numskulls and Billy Whizz. This particular strip is taken from the Sparky Book 1971; the Christmas books were usually finished at least a year before publication, and Baba and his guardian angel must have just sneaked in before the deadline to debut in the weekly comic and the accompanying annual in the same year.
In this particular strip, Ali's invisibility to the other characters is represented by making him transparent. Which seems fair enough. However, it appears very unlikely this method was used in any other strip.
For the vast majority of editions (this example is from the 1973 book),1 they did something more interesting -- having the outline of Ali and his speech balloons appear in colour:
