Fenn is narrow-shouldered and stooped, with thin wrists and a face folded into fine, maplike lines. Sparse white hair lies swept back as if even his grooming were an attempt to keep the evidence orderly. Three black ritual marks run vertically down his forehead, their edges blurred with age but still stark against his skin. He moves in abrupt, economical adjustments, as though constantly correcting his position against an invisible measuring grid. His jaw is slightly too broad for his gaunt face; when he eats, he chews with careful, even strokes, and when amused, his brief grin reveals small, crowded teeth. He looks frail until the room's shadows begin keeping time with him.

Fenn is detached enough to describe a catastrophe in measurements, yet privately keeps the names of the dead close and remembers small details about the living. With scholars he is exacting and unexpectedly patient; with grieving people he is awkward but tries to listen. Under pressure, curiosity makes him bold, while a sudden reminder of his first failed expedition can make him evasive and cold.

For generations, Fenn's family served as recorders for the Watch kept at the place locals call Gull-Can't-Be-Killed, a name earned by the scavenging birds that persist around its wind-scoured ruins. Fenn inherited the post of Master Observer and a chained folio of observations that reaches back centuries. As a young researcher, he accompanied an expedition toward the Final Spire. A ritual measurement failed, and the returning team found that one member had died hours before anyone remembered seeing him fall. Fenn survived by abandoning the attempt to save the man and recording the anomaly instead. The choice made him both more useful and less trusted. He has spent decades documenting death distortions, ritual measurements, and failed approaches to the temple, while privately maintaining correspondence with his estranged former apprentice, Tamas, who believes the research should be destroyed. Fenn's loyalty to Nerull is sincere, but his devotion to knowledge is stronger. Garran's death, Ilyra's injuries, and the destruction of the Continuance Engine have upended his accumulated models. He wants to question the survivors before the Watch seals the site, and his superiors want him to stop asking.