The Wearers of Purple are the only ones that wear purple robes, and the only available Wearer of Purple in Episode 1 is Mardath, as others have said. The beginning of the Prisoners section referenced says this:

If the characters haven’t run into any cult leaders yet, Nighthill takes them onto the parapet and points out what he means. This is an ideal time for everyone to catch a glimpse of Frulam Mondath[.]

While this may seem to imply that there are other, unnamed Purple Leaders walking around, the book is actually giving you the flexibility to allow your players to somehow end up seeing Mondath beforehand, perhaps in the streets. If you do allow them to see Mondath beforehand, then the module is just saying to not have Nighthill take them to the parapet to point him out, as the players already know what he looks like.

The Wearers of Purple are the only ones that wear purple robes, and the only available Wearer of Purple in Episode 1 is Mardath, as others have said. The beginning of the Prisoners section referenced says this:

If the characters haven’t run into any cult leaders yet, Nighthill takes them onto the parapet and points out what he means. This is an ideal time for everyone to catch a glimpse of Frulam Mondath[.]

While this may seem to imply that there are other, unnamed Wearers of Purple walking around, the book is actually giving you the flexibility to allow your players to somehow end up seeing Mondath beforehand, perhaps in the streets. If you do allow them to see Mondath beforehand, then the module is just saying to not have Nighthill take them to the parapet to point him out, as the players already know what he looks like.

The Wearers of Purple are the only ones that wear purple robes. The first part that talks about the Wearers of Purple says:

Leaders in the cult are known as Wearers of Purple, and they outrank normal cultists, but no formal grades exist within the Wearers of Purple.
--Hoard of the Dragon Queen pg 7

The sidebar in the second episode talks about a heirarchy in the rest of the cult, and names the Wearers of Purple.

There are only five wyrmspeakers, one per color of chromatic dragon. The current wyrmspeakers are Severin (red), Galvan (blue), Neronvain (green), Rezmir (black), and Varram (white).

Most of the cult’s operations are overseen by local leaders called Wearers of Purple for the ceremonial robes that they wear. All of the wyrmspeakers are Wearers of Purple, as are lower-ranking cultists appointed by the wyrmspeakers to preside over cult enclaves.
--Hoard of the Dragon Queen pg. 19

So, Wearers of Purple are only the five wyrmspeakers, and any local cultist leader, such as Mondath, that you happen to come across.

The Wearers of Purple are the only ones that wear purple robes, and the only available Wearer of Purple in Episode 1 is Mardath, as others have said. The beginning of the Prisoners section referenced says this:

If the characters haven’t run into any cult leaders yet, Nighthill takes them onto the parapet and points out what he means. This is an ideal time for everyone to catch a glimpse of Frulam Mondath[.]

While this may seem to imply that there are other, unnamed Purple Leaders walking around, the book is actually giving you the flexibility to allow your players to somehow end up seeing Mondath beforehand, perhaps in the streets. If you do allow them to see Mondath beforehand, then the module is just saying to not have Nighthill take them to the parapet to point him out, as the players already know what he looks like.