
Originally Posted by
Orophin
Is there anything supporting they're the same person? <...> he does appear in some cutscenes that don't appear to be in the past, such as the one where he's looking for Emerick on The Executioner or where he's talking to the head of the Blacksmithing guild. The last cutscene with him and Y'shtola also appears to be in the present, even though you do step back in the past with an Echo vision.
Yes, I believe that it's no less than 100% certain they're the same character. I admit now that I haven't been through the Marauder quests, and this is something I'll do at a later point if what is below doesn't convince you.
Let's start with his name. Roegadyn names are based on their ancient language, which is basically just made up of various Germanic influences. Many names are confirmed, others you can figure out easily. Rostnsthalmann's is among the confirmed, but for the sake of completeness, let's use our Admiral's name for an introduction to this concept.
Merlwyb Bloefhiswyn can be split into the ancient Roegadyn words merl (sea), wyb (wife), bloe (blue), fhis (fish) and wyn (daughter). These correspond to words from various ages of Germanic influence, mer (ocean), weib (woman, another word for wife; eg "my woman"), blau (blue), fisch (fish), and wyn, which I'm not entirely sure about but hesitate making connections to because I'm pretty sure it implies daughters are lesser and kind of thought of in monetary terms. Anyway, moving along...
Sthalmann's name comes from the German words sthal (steel) and (Proto-Germanic) mann (man). This is the name he lived under when he able to wear it with pride as Commodore of Limsa Lominsa. However, as Commodore, he had the Admiral assassinated and tried to take the position as his own without a fair Trident by colluding with pirates and stealing The Key. There's no recovering from that - and due to his being caught, he lost everything. His second in command, Reyner, was given his position, he was exiled from the Barracudas, and is thought of city-wide as trash. This is why he changes his name to Rostnsthal, which is made up of the words Rostn (rusty) and Sthal (steel). Thus, because of his shame, he has gone from Man of Steel to Rusted Steel.
Next, almost every echo you enter where Sthalmann is Commodore is triggered by Rostnsthal's presence. Shapeless Melody even underwent changes a while after release to add a tutorial, and in that tutorial we get a much better idea of where this echo event came from. Rostnsthal comes below deck and tells you that he's only seen a storm like this once before, and many people died that night. You go above deck, echo starshower begins, and Sthalmann is present. In this cutscene, you see him firing his weapon. When you enter the Musketeer's guild and take a firing stance for the current Commodore, Reyner (who was Sthalmann's number two), it reminds him of Sthalmann and you enter an echo where he recruits the Sanguine Siren spy. Reyner then tells you that you need a new mentor, as he knows that Sthalmann is a complete disgrace (rusted steel).
At the Blacksmith guild, Rostnsthal brushes past you to confront the guild leader, which triggers an echo where he is discussing the building of a ship to join the Seal Rock expedition, which he is told has been wiped out in this very conversation. We exit this echo, and Rostnstal is engaged in an argument with the guildmaster over whether or not to pay for this very ship, which sunk due to quality control issues stemming from his shoddy investment. When we first visit the Astalicia, we are punched by Rostnsthal, which triggers an echo when he, as Sthalmann, punches Emerick while "arresting" him on this ship some time earlier.
When we visit the ship out at sea, we enter an echo there, as well, except it's either Hob's or the current Commodore's. Neither pirate spy is on the ship at the time, which is why we have to make up an excuse for the Barracuda's when Hob gets arrested. This was all triggered by the sighting of the Serpent Reavers, who last appeared when Travachant attacked the ships. You never see Hob give an order from any of the three ships in this scene.
Take this back to the first time we speak to Baderon, where he tells us that we're not the first person to ask about sea serpents. This causes him to remember being interrogated by Y'shtola, who was only in town for two weeks back when Sthalmann was commodore. This can be shown by Baderon referring to the Admiral as a HE, which we know was some time ago because Merlwyb won the last legal Trident after Sthalmann's execution of the previous admiral after he successfully takes The Key with the help of both pirate faction spies.
The final Sthalmann echo is triggered by Sisipu's memories of her uncle. Sispiu fishes as the same place her uncle used to take her - a little tidepool where the Sahagin dare not go because of the shark toxin. This is where the three colluded together. This is where it gets hairy.
We know from Rostnsthal's echoes how Sthalmann planned to steal the key, from the Commodore's echoes how he used the pirates to do it, and from Sispu's memories where they met to do it. In the last Commodore echo, we're shown that Sthalmann went to Sispu's fishing spot to confront Y'shtola. When we get there, the only person around is Blackburn.
I believe it his he that triggers our last Sthalmann echo. Blackburn suspects us of having the echo, and is waiting for us at this location. After he tells us that he's sure we're going to understand soon, we pass out and enter the culmination of this story, where Travachant makes off with the key, and we wake up back at Baderon's. I think it was Blackburn who first discovered what happened here, as he and Milifinia already know about Y'shtola and her loss of the key.
Again, I could be wrong about any of this, but I'm damn sure that Sthalmann is Rostnsthal a few years(?) back and that we don't meet Y'shtola in real time throughout this entire thing.
I just can't figure out exactly how many damn years ago it was.
Also, just look at them - same haircut, same beardline, same eye color, same skin tone, and the same eye scar, which you can see the bottom of sticking out of Rostnsthal's eyepatch (might need to right-click "View Image" and look at the big versions to see the scar tip).