Klaus is a character, who was once an East German Olympic ski-jumper until his brainwaves were switched with that of a goldfish in the 1986 Winter Olympics by the CIA to prevent him from winning the gold medal. Klaus' human body with the goldfish brain headed for a nearby stream, and was later found frozen and allowed to decay by the CIA in "Da Flippity Flop".
Klaus took up residency with the Smith family, where the CIA ordered Stan to have him pose as their pet, a fact Klaus has still apparently not come to terms with. If something is done to him unexpectedly he gets very annoyed and swears revenge.
A recurring joke of the series is that Klaus' dark behavior, is due to him being German. In "Deacon Stan, Jesus Man", when he tricks Roger into eating Francine's potato salad, Roger is horrified asking Klaus why, to which Klaus responds "I'm German, it's what we do." Klaus will often drag out the revenge until he has forgotten about it such as in "Surro-Gate", but he hates to be interrupted during his 'snooze'.
Klaus' mind has apparently enhanced the physiology of his goldfish body in addition to the ability to talk. Klaus is apparently able to breathe just fine as long as part of his body is submerged in water, which is often shown when he drags himself along the floor in a shallow water dish. He may actually have the ability to survive in almost any type of liquid, since he once hid in a thermos filled with coffee in "Francine's Flashback", was frozen in his fishbowl when Karl Rove passed by in "Deacon Stan, Jesus Man", was congealed in jello when dared by Hayley in "Frannie 911", survived being put in a freezer by Stan, in "Brains, Brains and Automobiles" by filling his bowl with vodka, and was put in a washing machine by Steve in "Roger Codger" and by Roger in "Great Space Roaster". Ordinary goldfish will immediately die if placed in any liquid other than water, or superheated water such as that found in a washing machine. However, in "Hurricane!" he claims that salt water "burns" him when attempting to swim in it. In "Anchorfran", he fell screaming into a boiling pot while assisting Francine with dinner, however, he shortly reappeared and indicated he was enjoying himself.
He also claims to have had his brain inhabit two sharks and a monkey in "Of Ice and Men", but this cannot be confirmed as true, as Klaus seemed to be sarcastic, and in a tone of voice that is ordering his grandson to be quiet and go to bed.
Roger and Steve once agreed with one another that Klaus was a psychopath when they thought he poisoned Roger's chocolates as revenge for a prank and looked over at Klaus who watching them from his bowl singing to himself with an evil glint in his eye such as in "Surro-Gate". Klaus also encourages Francine to kill herself in "National Treasure 4: Baby Franny: She's Doing Well: The Hole Story".
In "Finances With Wolves", Klaus managed to temporarily become human when he switched his goldfish body for that of a black man's and almost ran off with Francine, but the body was mortally damaged in the aftermath of an explosion at the mall. Deciding to spare his life, but also to make sure he never try to seduce Francine again, Stan transferred Klaus's brainwaves into another goldfish's body; Klaus had flushed the previous fish's lifeless body down the toilet. This episode also gave an indication of his personality, while Stan dove to save Francine from the blast, Klaus was more concerned in saving his new human body, which showed that Stan cared more about Francine than Klaus. However after this, Klaus's obsession with Francine has largely tapered off. Showrunner Brian Boyle said that this was due to more closely matching Klaus' personality to that of Dee Bradly Baker's.
In "Of Ice and Men" he is shown in the year 2045, inexplicably he once again has a human body. Klaus has apparently been human again for quite some time, and married to an unnamed woman for many years, as he has fathered at least one child who has lived long enough to grow up and have a child of his or her own, meaning Klaus has a grandson in this future. It was not specified whether Klaus will be the maternal or paternal grandfather of that little boy. The majority of the episode consisted of a story Klaus told his grandson about the past.
During "Da Flippity Flop", Klaus discovered his old body had been found but Stan refused to help him reclaim it. After Stan is forced by his family to take Klaus, they find that Klaus' body is slowly decaying. An enraged Klaus knocks Stan out and takes his body, Klaus subjected Stan's body to extreme punishment and wallowed in the latter's suffering when the family permit it since Stan had it coming. However, after they discover Klaus will not return the body they try to stop him but Klaus escapes and goes to perform a deadly ski move. At the end, he fails and his old body is destroyed. Klaus is returned to the fish body, though a now sympathetic Stan vowed to ensure Klaus becomes human again but doubts he'd follow through.
In "No Weddings and a Funeral", Klaus took the body of a nun, sometime after leaving the Smith Family. Fed up with the family's abuse and mockery, Klaus would go onto bludgeon a nun and take her body to host his own funeral. Roger saw through this, with Klaus revealing it was a twisted ploy to get the Smiths to show much they much they appreciated but the plan fell through. Klaus abandoned the nun and put the Smiths in his old fish body along with himself. After a fire broke out and the family escaped, the Smiths all realized they needed Klaus to mock because it kept them whole.
Klaus and Roger's relationship is best described as a sibling rivalry. At times they hate each other and at other times they get along well. Roger is seen to be incredibly spiteful toward Klaus, saying things like "You can't participate Klaus, I hate you. I say that not out of anger but as a fact" in "Great Space Roaster" and "It's like you want to be kicked out of this family" in "The One That Got Away". Klaus does things to get Roger into trouble for his amusement, like when he tricked him in to eating Francine's potato salad in "Deacon Stan, Jesus Man". When a horrified Roger asked why Klaus would do something like that he just laughed and said "I'm German, it's what we do." In "A Piñata Named Desire", after Klaus made a comment about acting, Roger simply picked up Klaus's fish bowl, threw it across the room, smashed it against the wall, and walked out of the room as if nothing had happened while the fish lay gasping on the floor.
Although Roger has claimed to hate Klaus in the past, they do have a bond and are capable of being civil to each other such as when they head to Europe together in "Red October Sky". Further evidence of this can be seen in "Pulling Double Booty", as Roger and Klaus are quite content to watch a movie together and show annoyance when they must pause it as Stan intrudes with the cookie dough. Another example is that Klaus is often shown in the attic with Roger as he tends his fake bar in many disguises while in "Live and Let Fry" Roger dresses up as Klaus, the human, to help him get access to a family member's will. In the end it turns out to be a trap to lure Klaus out of hiding and Roger gets beaten up due to the fact that Klaus owned some money to the East German Mafia. However, Klaus stays with him and keeps him company during his recovery, making sure Roger has sufficient pain medication to help him through it. Klaus was also the only member of the Smith family to remember Roger's birthday in "1600 Candles" and the alien seemed genuinely touched that he had bothered to prepare a birthday surprise for him.
Klaus and Roger engage in a sexual relationship in "Kloger", but Klaus breaks it off when Roger starts to want to act like a real couple, as Klaus was only interested in the taboo aspect of sneaking around the family for their relationship.
Klaus has a strange obsession with the Disney film Beauty and the Beast. His view is that without Mrs. Potts; whom Francine played in "Stan of Arabia", the Beast would have killed himself years ago.
Klaus often has made up conversations and fantasies of situations. He could also be rather selfish and although he was attracted to Francine he did not attempt to save her during "Finances With Wolves", being more concerned with saving the human body he was currently residing in. However after Stan transferred him back into a goldfish's body, Klaus appeared to have lost some of his obsession with Francine.
Klaus often becomes caught up in make-believe or delusional states. This includes pretending to call people on an imaginary cell phone in "Bar Mitzvah Hustle", or driving by making motor sounds to a hair/nail salon for an appointment in "Man in the Moonbounce". In "Oedipal Panties", he convinced Steve to hold him so he could infect him with Ick, a fish disease, thus forcing Steve to spend time with him, while his other friends avoid him, out of fear of being infected.
In "Haylias", Stan asks Klaus for advice and he says "No. You only turn to me when there is no one else to turn to" and keeps reading. Indeed, Stan appears to secretly want Klaus dead, having had him mounted on a board in his personal heaven in "Rapture's Delight" and shot by the secret service in an extended dream sequence in "An Incident at Owl Creek". Though despite this they share a bond too; In "The Vacation Goo", Stan quite happily dumps his family into the goo and when Roger pulls the plug, they discover Stan watching sports with Klaus, both of whom are dressed in supportive team gear.
Klaus does seem to share a genuine friendship with Hayley, along with the ability to communicate telepathically with her at any given time in "Merlot Down Dirty Shame". Perhaps one of the most revealing moments of this is in "Choosy Wives Choose Smith", when Hayley announces to her mother she is going on the road for 15 weeks with Klaus to follow My Morning Jacket, after brief quizzing Francine responds with, "Okay, have fun you two!". Other examples can be found in many episodes. In "Dungeons and Wagons" for instance clearly shows the two working as friends quite gleefully after plotting against Steve. In "Man in the Moonbounce", Klaus mourns the fact that he can no longer go for a hair cut, and as such Hayley puts a lot of thought and effort into making a small wig for Klaus which she then styles for him in a pretend salon station much to his joy. And in "Oedipal Panties", Klaus tried to infect Hayley with his ick so that she would only have him to spend time with but his plan backfired when she revealed she had herpes. It also seems the Klaus has some sexual interest in Hayley. When Hayley gets a henna tattoo in "Frannie 911", they get into an argument which they decide to settle with a game of "Dare or Dare". Klaus takes a crack immediately and dares Hayley to kiss him. In "The Unbrave One", Jeff asks if anyone wants to have sex with Hayley because he was too tired; Klaus raised his hand with a smile on his face.
- Klaus' last name of Heisler is based on the writer Keith Heisler's. Subtitles in both "Live and Let Fry" and "For Whom the Sleigh Bell Tolls" give his name as "Heissler" but a book he shows Francine in "The Boring Identity" spells it as "Heisler."
- In "Bar Mitzvah Hustle", when Stan walks off the set, Klaus can be seen in his dish in front of a green screen. He and the dish are huge implying that he is actually big and is inserted into the scenes. The DVD commentary mentions the scene was at one time written for Klaus's voice actor Dee Bradley Baker to emerge from the fish costume.
- Klaus has a tendency to break the fourth wall, often making comments about his own lack of activity in a given episode.
- It was revealed in "The Most Adequate Christmas Ever" that Klaus' Grandfather was a train conductor at the Auschwitz zoo.
- In "Of Ice and Men", Klaus claims that he was roommates with Rick Schroder, although this will likely happen in the future. Another explanation is that Klaus could have been playing a joke on his grandson akin to the "two sharks and a monkey" comments.
- Klaus has a fetish for pregnant women as shown in "Oedipal Panties" when he is searching the phone book and states "she must be blond, stacked, willing to get down with a fish and nine months pregnant".
- Klaus can communicate telepathically with Hayley, as shown in "Merlot Down Dirty Shame".
- In "The One That Got Away", Klaus teleports to another world in a puff of smoke, a few seconds later he returns and claims he had been gone for 60 years and become the other world's king, this would make Klaus near 90 years old depending on how old he was when he was turned into a fish.
- In "Hot Water", it was said that Stan fixes Klaus a daily omelet.
- Klaus reveals that he writes lurid fiction about Stan and Francine in "The Boring Identity".
- It is implied that Klaus does cocaine in "Vision: Impossible".
- Often when appearing as a human, Klaus' head or hair will resemble a fish in some fashion, such as an old man in "Of Ice and Men", a young University student in "My Affair Lady", or the character Tchochkie Schmear in "Tearjerker".
- Conflicting with being an Olympic skier in 1984 and being forced to live with the Smiths since, Klaus claims to have gone to Arizona State University in the 1990s in "The Life and Times of Stan Smith".
- Klaus claims to be a Green Bay Packers fan in "Kloger".
- When Klaus is granted citizenship by Stan in "One Fish, Two Fish", he chooses the name of 'Ace McNasty.'
- His appearances in the Stephen Squirrelsky and Friends' Movie Spoof Travels are Sleeping Beauty (Justin Quintanilla's Style), Aurorastasia, Despicable Me (Justin Quintanilla's Style), We're Back!: A Jungle Animal's Story, Jimmynocchio, and The Wizard of Oz (Uranimated18's Style) and will even be in more movie spoof travels.