Squeaky, the narrator of the story, is called Mercury by the big kids in her town because she is “the swiftest thing in the neighborhood.” Her real name is Hazel Elizabeth Deborah Parker, and people call her Squeaky because of her squeaky voice. However, she is also faster than everyone she knows, except for her father, and this explains her second nickname.

Mercury, in Greek mythology, is the messenger god, known for his extreme speed and swiftness. He has winged sandals that help him to travel at top speeds, delivering messages for Zeus, his father, and he flies all over Greece and Mount Olympus as required. For this reason, then, it makes sense that Hazel, or Squeaky, would be called Mercury by the older kids.

Mercury is also the name of the planet with the fastest revolution around the Sun in our solar system. It takes only eighty-eight days for the planet to complete one orbit, and it moves at almost twenty-nine miles per second, which is faster than every other planet of those that orbit our Sun. This is likely why it was named after the fastest Greek god.

So, we have lots of cultural associations with the name Mercury, and all of them have to do with speed. Thus, Hazel/Squeaky is nicknamed Mercury due to her ability to run so fast.

Why do the older kids call Squeaky "Mercury" in "Raymond's Run"?

In ancient Roman mythology the god Mercury often performed the role of divine messenger. If the gods had important messages they wished to convey to mortals down below they would entrust the fleet-footed Mercury to deliver them. Mercury was often depicted in countless paintings, statues, and mosaics as wearing winged sandals. This was a way of emphasizing how quickly he could travel.

It's not surprising, then, that Squeaky, who describes herself as the fastest thing on two feet, should be compared to this legendary figure from antiquity. For good measure, Squeaky also refers to herself as "Miss Quicksilver." This is appropriate for two reasons. First, because quicksilver is another name for the chemical element mercury; and second, because it emphasizes Squeaky's speed as a runner.