Hugo Ashdown is assumed to be one of The Four Witnesses. Little is known about him. He was probably involved in Wright's experiments, as he is seen entering a portal that leads to the Veil at the start of Chapter 7 (The Reunion). There are theories that Lena Ashdown, a patient who died at St. Gall. was a close relative.
Hugo may have been the fourth witness at the boarding school. In Devitt’s memories (Chapter 2), the fourth witness looked a lot like Hugo in Chapter 7 (especially the hair). The note that Ernest swallowed had four names on it: Ernest Glynn, Jeremiah Devitt, Alexandre D., and Hugo Ashdown. Hugo’s name was the only one that was crossed out. This note may imply that Hugo was one of the original Four Witnesses and that he was presumed dead.
When Wakefield explored the caverns below Wright Manor, he found shredded clothes with the initials “H.A.” printed on them (Chapter 6). The cold open of Chapter 7 implied that Hugo was the subject in Wright’s experiments. In the second scene of Chapter 7, Wakefield described the monster in the cave. He said that its screech had a human sound to it, like a desperate cry for forgiveness. The fact that one scene
followed the other might suggest that they were both about Hugo, before and after his transformation. The fisher in Wickport saw a monster that was “big and hunched over, like a crouching man.” That description seems to fit the creature that Wakefield found in the cave (Chapter 6).
If Hugo was the subject in Wright’s experiment, he was probably a member of The Playwright. If he had known Alexandre since school, he might have been one of the Playwright’s first members. Devitt found an undeveloped photograph in Alexandre’s home, which turned out to be a picture of Anthony, Alexandre, and a third shadowy figure. The shadowy figure seems to have short hair in the back, like Hugo (Chapters 4 and 7). When Devitt showed the picture to Alexandre, he started breathing heavily. Maybe Alexandre recognized Hugo in the photo and was overcome with guilt (or horror) at what had happened to him.
During Wright’s hypnotism session, he described a cave. In one cavern, he said there was a piano, and that something was standing next to it. Wright couldn’t clearly see what it was (or maybe he couldn’t see its face clearly). Deeper in the cave, there was a pit that contained something terrifying. Later, when Wakefield explored the cave, he heard someone playing the piano. Maybe Hugo was the creature and he was playing the piano. But in that case, what was the terrifying thing that Wright said was in the pit? If Wakefield had continued exploring the cave, would he have found another creature that was much more dangerous? Or maybe that creature had escaped and killed Wright in his home? Or maybe the “pit” was simply a metaphor for the Veil, and the terrifying thing was the Simurg. (Though Wright never mentioned seeing a bird’s eye.)
According to the St. Gall hospital records, a patient named Lena Ashdown died on November 27, 1891, of osteoarticular tuberculosis (tuberculosis of the joints). The record states, “During the autopsy we found some strange black marks on his fingers” (Chapter 2). [Update: TGK_RDM has clarified that the text should say "her" instead of "his".] It’s unclear whether Lena and Hugo are related. I don’t know what the strange black marks could be. I also can't explain how someone who was injected with serum could present symptoms that are associated with tuberculosis. Is it possible that she wasn't injected at all?