Professor J.S. Heinrich Ibsen is the mad scientist of Jumanji. He invents dangerous Steampunk-like machines, but he doesn't even know why he does it. Ibsen is tasked to "work" for Jumanji, creating dangers and nightmare machines like poisoned acid-spitting frogs and battle-armored rhinos. He sends daily reports to "Jumanji" itself which broadcasts throughout the jungle.
Ibsen dresses in a scientist longcoat and driver turn-of-the-century style getup, and his car, factory and creations (apparently) all run on steam punk machinery. He wears googles that cover his eyes, and a tall top hat whenever leaving his factory.
Ibsen disregards Jumanji's natural living animals and people as "organic trash" and adores robots as his "babies". Although he tends to build killing machines, he has no interest in killing with his own hands (or weapons) unless The Stalker tells him to, as in The Gift. Though mostly indirectly, Ibsen has tried to kill Alan countless times before Alan even learns of his existence, after he saves the trio from Verbilangs in his car. In retrospect, his admiration for Alan's talent, intelligence and capacity to survive, leads to the professor questioning if Alan is actually a robot made by Ibsen rather than "organic trash". Ibsen appears to have a soft spot for children, since he never appears to directly want to kill any of them.
Ibsen can make robotic duplicates for everything and everyone and is also revealed to be a robot skeleton under a human-looking exterior, Terminator-style, which begs the question of who made him in the first place. But since he appears again several times after being "killed" in his debut episode, it may have been another duplicate that took his place the first time, since he has been known to build very convincing copies of people and animals. Later on, Ibsen even builds duplicates of Alan and Peter and even cackles about the possibility of duplicating everyone else to take over the outside world. He can active machines using a voice control box, invoked and exploited in "Robo-Peter", where the clue is about making Ibsen's creations turn against him.
Despite creating machines that can kill, Ibsen is a very nice and polite man who will accommodate guests inside his factory, until he builds a machine that can kill in a horrible way just because that's what he does. And despite seeming like the maker and intelligence running Jumanji at one point (even claiming to "be" Jumanji), Alan is understandably mad when he learns that Ibsen made many (if not all) the creatures that have tried to kill him over the years. When asked if he also built the conventional jungle animals like Hyenas and Crocodiles, he disregards them as "organic trash" and denies it. Ibsen runs on a tight schedule. At 20:00, he will always stop what he is doing and broadcast his daily report on Jumanji, even though no one seems to be listening to it. In-universe, he is actively modifying Jumanji to be this, by designing, building and adding in new bizarre, deadlier creatures to the hungry jungle. He has CCTV recording what everyone and everything does in Jumanji, and has detailed records of what everyone and everything did - and will do. He is one of the few genuine human inhabitants who, the same reason as Van Pelt and Slick, is aware of Jumanji's realm being within a game and refers to it as such, but during a power trip from stealing Judy's laptop and gaining even higher levels of control over Jumanji, he yells that he is now Jumanji altogether.
Ibsen lives and works in his giant factory is in a plain subjected to tidal waves. The water reaches up to the roof, but don't worry, he has a pressurized chamber to take refuge in when that happens. If he is defeated while inside his factory, the factory is guaranteed to come crashing down. It is also guaranteed to be up again in the next episode, however. Out of it, Ibsen is the first enemy that has no counterpart in a typical Jungle Opera, and the first hint that there is a mechanical, off-stage side to Jumanji and a mind running it all, even though it is not Ibsen himself.
Ibsen, like everything else in Jumanji, was absolutely terrified of the imposing Stalker and did whatever he was told by the grim reaper, even working with Trader Slick and Van Pelt against their bickering to hunt and kill the trio to prevent Peter's threats to destroy Jumanji from coming true.
A self-described "Master Builder", Ibsen is seemingly capable of building any machine. Although Ibsen clearly enjoys his work, he estates several times that building deadly machines is just what he does and that he merely works there. He doesn't actually try to explain why he should do that or who is employing him. His creations tend to combine features from different animals and plants.
- Ibsen claims several of Jumanji's inhabitant wildlife are his creations, down to basic animals like Hyenas, Panthers, Crocodiles and more advanced things like carnivore plants, armoured hippos, acid-tounge frogs and Verbilangs which are cave-dwelling, photophobic, hairy, flying piranhas by illuminating them with his Steam Punk car's lights.
- Despite Ibsen's love for his creations, Alan notices that they are dumber and weaker than the animals they mirror and are supposed to supersede.
- Carnivore plants have been present since the first episode (and were also in the 1995 movie), and he later sends an armoured hippo after the heroes (but pieces of one can be seen earlier in his workshop). An acid-spitting toad appears in the following episode No Dice. At one point, he builds flying monkeys very similar to those in The Wizard of Oz.
- In Air Judy, Ibsen has a vaguely whale-shaped zeppelin, which is revealed to be stealing eggs from the Jumaki Tribe so that he can use their yolks to fuel a new design of battle armored Hippopotamus.
- He does not know what a plane is.
During another visit to Jumanji, sunset approaches and they must escape the cave as "Verbilangs" come out and attack at night. Professor Ibsen gives them a lift to his laboratory, where Alan sees all the dangers he's faced in Jumanji. Ibsen reveals he creates the dangers in Jumanji. He sends a daily report to "Jumanji" itself, which broadcasts throughout the jungle. Judy plugs a large mainframe computer into her laptop which gives her laptop the ability to analyze everything in Jumanji and predict the future, revealing that within seconds a flash flood will hit. The trio escapes to high ground, and are chased up a tree by mechanical hyenas. Judy tries to get finds Alan's clue, just as Ibsen knocks the tree down to steal the laptop. He uses it to create even more monstrous creatures, reform damaged ones, and reshape the terrain to make it more dangerous. The trio race through the jungle while he is busy doing his daily report. During the report he makes a speech about how he now rules Jumanji and Peter figures out their clue. At his lab they try stealing the computer back, but his machines attack, and then the computer overloads creating a cyclone. Peter realizes their clue ties in to how to stop the computer. He asks it "why", which it tries to answer but cannot since it has tno context and it shuts down. Ibsen shuts down too, revealing that Ibsen is also a machine. His factory is destroyed and the kids return home.
Peter vows in anger that he'll destroy the game if Alan dies. Without noticing, The Vengeful Grim Reaper, Stalker overhears with the microphone, disguised as a flower plant. Stalker vows to kill the children and Alan in order to protect Jumanji, enlisting Slick, Van Pelt, and Ibsen to do the task.
Judy ventures to Slick's looking for a cure. Instead, he gives her "Jumanji Tracking Juice" in exchange for her shoes which reveals her footprints and allows the evil trio to hunt her. The three apes deplete the juice before the villains find her. Peter journeys to the Manjis to meet Tribal Bob, who is being watched by the villains. He does plant a small statue on Peter's head without Peter noticing, who is then stalked by the villains. He manages to elude them, and Judy finds the statue, of a mosquito. They realize a Jumanji mosquito can suck out the venom from Alan's veins. But upon capturing one, the villains attack, armed with weaponized vehicles from Ibsen. The kids elude them, and Tribal Bob and the Manjis carry off the villains.
He calls himself a "Master Builder of Jumanji." This is a reference to the Norwegian playwright Henrik Ibsen and his play "The Master Builder". Though his namesake, Henrik Ibsen, was Norwegian, Ibsen uses the German spelling, Heinrich.