This could potentially go both ways.

The description for Scrying includes (emphasis mine):

You can see and hear a particular creature you choose...

As a DM, I would interpret this as particular = specific, so in this case your NPC/persona wouldn't be a valid target. This could lead to some mystery for the PC, as they try to figure out why they can't scry this "individual."

Alternatively, you said the people in question rotate duty as this persona, so you could have the spell choose whomever is on duty as that particular person for that day.

Another option is to roll a die for whom the spell picks. There are 3 people, so roll a D6 and see who you get. This could get especially interesting if the spell picks someone NOT on duty as this persona, when the PCs know the vigilante is busy somewhere else.

Yet another option is as PJRZ mentioned below, where the PC has to describe how they know of this vigilante, and whichever person did what the PC describes is the target of the spell.

It is up to the DM.

The ultimate answer here is "it's up to you," but let's explore it a bit since this could go a number of ways.

The description for Scrying includes (emphasis mine):

You can see and hear a particular creature you choose...

As a DM, I would interpret this as particular = specific, so in this case your NPC/persona wouldn't be a valid target. Xanathar's Guide to Everything offers an optional rule for spells with invalid targets as the following:

If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn’t attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

If we choose to use this rule, the straightforward "answer" in this case is that the spell simply fails since there isn't a single valid target.

However, as a DM, I might interpret this situation a number of other ways:

I might choose whomever is on duty as that particular person for that day, or a random member of this group.

Or, as PJRZ mentioned below, the PC might have to describe how they know of this vigilante (since scrying allows for a target "you have heard of"), and whichever person did what the PC describes is the target of the spell.

#This could potentially go both ways.#

The description for Scrying includes (emphasis mine):

You can see and hear a particular creature you choose...

As a DM, I would interpret this as particular = specific, so in this case your NPC/persona wouldn't be a valid target. This could lead to some mystery for the PC, as they try to figure out why they can't scry this "individual."

Alternatively, you said the people in question rotate duty as this persona, so you could have the spell choose whomever is on duty as that particular person for that day.

Another option is to roll a die for whom the spell picks. There are 3 people, so roll a D6 and see who you get. This could get especially interesting if the spell picks someone NOT on duty as this persona, when the PCs know the vigilante is busy somewhere else.

Yet another option is as PJRZ mentioned below, where the PC has to describe how they know of this vigilante, and whichever person did what the PC describes is the target of the spell.

This could potentially go both ways.

The description for Scrying includes (emphasis mine):

You can see and hear a particular creature you choose...

As a DM, I would interpret this as particular = specific, so in this case your NPC/persona wouldn't be a valid target. This could lead to some mystery for the PC, as they try to figure out why they can't scry this "individual."

Alternatively, you said the people in question rotate duty as this persona, so you could have the spell choose whomever is on duty as that particular person for that day.

Another option is to roll a die for whom the spell picks. There are 3 people, so roll a D6 and see who you get. This could get especially interesting if the spell picks someone NOT on duty as this persona, when the PCs know the vigilante is busy somewhere else.

Yet another option is as PJRZ mentioned below, where the PC has to describe how they know of this vigilante, and whichever person did what the PC describes is the target of the spell.

#This could potentially go both ways.#

The description for Scrying includes (emphasis mine):

You can see and hear a particular creature you choose...

As a DM, I would interpret this as particular = specific, so in this case your NPC/persona wouldn't be a valid target. This could lead to some mystery for the PC, as they try to figure out why they can't scry this "individual."

Alternatively, you said the people in question rotate duty as this persona, so you could have the spell choose whomever is on duty as that particular person for that day.

Another option is to roll a die for who the spell picks. There are 3 people, so roll a D6 and see who you get. This could get especially interesting if the spell picks someone NOT on duty as this persona, when the PCs know the vigilante is busy somewhere else.

#This could potentially go both ways.#

The description for Scrying includes (emphasis mine):

You can see and hear a particular creature you choose...

As a DM, I would interpret this as particular = specific, so in this case your NPC/persona wouldn't be a valid target. This could lead to some mystery for the PC, as they try to figure out why they can't scry this "individual."

Alternatively, you said the people in question rotate duty as this persona, so you could have the spell choose whomever is on duty as that particular person for that day.

Another option is to roll a die for whom the spell picks. There are 3 people, so roll a D6 and see who you get. This could get especially interesting if the spell picks someone NOT on duty as this persona, when the PCs know the vigilante is busy somewhere else.

Yet another option is as PJRZ mentioned below, where the PC has to describe how they know of this vigilante, and whichever person did what the PC describes is the target of the spell.