Readers use names to tell characters apart, obviously.
Authors sometimes complicate this by using the same letter to start character names, using similar-looking or similar-sounding names.
“Neville Longbottom” is a great example of a name that suggests a trait without being too ridiculous.
For example, my Agent Black is also known as the Manhattan Mangler.
For example, we aren’t meant to understand Judge Dredd and The Punisher as even-headed men of justice.
If those guys were named John or Mike, we might feel confused and disorientated.
If your work is long enough that you name 15 characters, then you can start sharing the letters of minor characters who appear in completely different parts of the book.
Also, try to mix up the number of syllables in your character names. A character’s name should establish or at least suggest a defining trait of the character.