Marethyu is a central character in The Secrets of the Immortal Nicholas Flamel. You can read more about him here, but that Wikia page contains major spoilers for the series, so follow that link at your own peril.
Marethyu is, among other things, Death:
Marethyu was said to have gone by many different names prior to his appearances in the books - however, all of these names meant Death.
He is also directly involved in many occurrences throughout the books that involve his dealing death, so the appellation is appropriate.
His "signature piece" is his hook, which replaces one of his hands. Spoilers:
That hook is constructed from the Four Swords of Power: Joyeuse, Excalibur, Durendal, and Clarent. The swords are each connected to one elemental power (earth, air etc). It is also called the sword, or hook of "Aether," and confers to its wielder power over time.
Was this hook intended to parallel the Grim Reaper's scythe?
The personification of Death in many cultures often wields a farm tool with a curved blade. Even Death of the Endless is occasionally shown with a scythe. I've seen many "fan arts" of Marethyu's hook online (as far as I could tell, none official), and most look something like Captain Hook's hook, and not much like a scythe. I don't remember any of the book's descriptions of this hook's appearance, but it has to appear as something that can fit on the end of one's arm, which obviously means that though it is a curved blade, it does not appear exactly as a scythe does.
Physical appearance aside, is there any indication from the books that Marethyu/Death's hook is supposed to be a scythe? A trivial Google search didn't turn up anything relevant.