Can Sildar and this character recognize each other in Lost Mine of Phandelver?

Question

When he's been rescued, Sildar asks the party to investigate

Tresendar Manor because he has learned that his colleague Iarno Albrek disappeared while exploring the area about two months ago.

Sildar himself might go with the party (it was his friend and his primary mission, after all). Unfortunately, the LMoP book says nothing (or I didn't find it) about what happens if

Sildar meets Glasstaff in his quarters and recognizes him as Iarno Albrek. Will he recognize him at a glance? Will Iarno recognize Sildar? How should both react?

Answer 1

Its entirely up to you

If you had written the adventure yourself this is something you may have decided when you wrote it, or it may be something you think about as the characters develop in playing, or a bit of both.

Personally, as there is no indication that Glassstaff has changed his appearance, there would be no reason that Sildar would not recognise him. Your role as DM is to play both NPCs reactions in accordance with your take on their personalities.

By the way, this is not something you can be wrong about.

Answer 2

Since the Lost Mines of Phandelver book does not specify this, it is your decision to make.

If Sildar recognizes Iarno it would most likely lead to more roleplay with diplomacy than fighting, as Sildar would probably want to ask questions as to why Iarno left and based off of Iarno's personality I think he would want to respond.

That aside, I would reccomend Sildar recognizing this person because

not having Sildar recognize Iarno will break immersion somewhat as it would seem a bit absurd that Sildar wouldn't recognize the man he was looking for when seeing him face-to-face.

Answer 3

The character has changed his name, but not his appearance

In Phandalin Locations, the Quest Finding Iarno says:
Unknown to Sildar, Iarno created the Redbrands, installed himself as their leader, and took the alias Glasstaff to conceal his identity.
and
Sildar describes Iarno as “a short, dark-bearded human wizard in his thirties.”
...
While the flavor text for the party's encounter with the character says:
Sitting at the desk is a short, dark-bearded human male in robes, studying a tome. [Redbrand Hideout - Area 12 (Glasstaff's Quarters)]

Sildar's description of the character exactly matches the description of the character when the PCs meet him. It is clear that the character has made no effort to change his appearance, but only his name.

In Sildar's Background, we learn:

After questioning several locals, Sildar learns that Iarno Albrek, a fellow member of the Lords’ Alliance, disappeared while exploring the area around Tresendar Manor about two months ago, shortly after arriving in Phandalin. Sildar asks the characters to investigate the manor and the surrounding area to find and bring back Iarno—or what’s left of him, if something killed him.

So we know that the character passed briefly through the town and then 'disappeared'. It has not been necessary for him to change his appearance since no one in town has seen him since.

Putting all of this together, Sildar and the character last saw each other a few months ago, and neither of them have changed their appearance since, although the character has taken on a new name. They should both immediately recognize the other on sight.

What happens next?

The adventure is quite clear on how Sildar will react:

In Phandalin Locations / Quest - Finding Iarno:
Once he learns the truth about Iarno, Sildar expresses a desire to have the wizard captured and transported to Neverwinter to face the judgment of a higher authority.
...
While in Overview of Act 2: Redbrand Hideout : Developments:
If Iarno is taken into custody, Sildar Hallwinter arranges to have the wizard incarcerated in the townmaster’s hall until he can be safely transported back to Neverwinter.

So we know very well how Sildar will react to this discovery.

The character's reaction, however, will depend on the situation.

We are told that Iarno is evil and that he wants to "line his pockets" and "secure his own position in town". We are also told that no other member of the Lord's Alliance knows his true identity, so he would likely want to keep things this way. If he wins a fight with Sildar present, he might offer Sildar a chance to join him, but he will not let him report back to the Lord's Alliance and that might mean imprisoning or killing him.
If he loses a fight, we are told that:
If he is reduced to 8 or fewer hit points and has no avenues of escape, Iarno surrenders. He values his life more than anything, and he remains a model prisoner in the hopes that the Black Spider will somehow learn of his predicament and “arrange for his freedom.”
In fact, if Sildar is not present when the party defeats Iarno, he might even reveal his identify to them and remind them that Sildar will want him alive.