Leo Newman (or Teo Dujardin, in the original French) is a human boy of twelve years old who is the reason the series he's in is called My Friend Marsupilami. Much like his father, Leo can be quite stubborn and, to a lesser extent, he inherited some of his dad's techno-savy with a prized collection of toy robots. Like his mother, Leo is brave and curious about animals, thus becoming best friends with Marsupilami, and eventually, the apprentice yamatunga.
In the show's very first episode, "The First Encounter", Leo and his family had just moved into the jungle in a refurbished dock-house by the river, but since there are no other human families in the jungle (let alone children aside his little sister, Zoey), Leo was lonely, and quite bored, as he unwittingly made an impact on Mars (or "Marsu" as the family grew to call him) when that marsupilami heard the commotion of his new naighbors and stopped by to investigate... in fact, when Mars told his family about the humans in the jungle, the part about the "lonely little boy" was one of few parts that he didn't exagurate! The next morning, when Leo was having bread and strawberry jam outside, he unwittingly invited Mars over for breakfast, and their first impression on each other, to put it politely, was a "scream" (liturally), but Leo's panic was canceled out by laughter as Mars tickled him while licking the strawberry jam off his new friend's face... a detail that got Leo's father, David, sure that marsupilami's are dangerous creatures and forbids Leo from going anywhere near Marsu until animal-expert Amanda (David's wife/ Leo's mother) can prove that marsupilamis are not dangerous.
Later that day, Leo babbles on about how his dad's overprotective instincts is making his life lame, but then sees Marsu playing around while catching and eating piranhas while swimming in the river. Laughing at the sight, Leo admits he knew all along that Marsu wasn't a monster, but none-the-less, he keeps their newfound friendship a secret, especially from his father. While the rest of the family was taking the boat with Mr.Didi to get grocceries from the near-by town of Chiquito, Leo gets lured into the jungle by Marsu when the long-tailed rascal steals the best robot in the boy's collection, but what starts out as a challange of keep-away turns into a playdate that strengthens the bond, but as it gets late and Leo heads home, Brenham Backalive notices the boy and calls his parents, then gives a paranoid speach to the Newman family that the marsupilami is a dangerous animal and advises they set up traps, but Leo and his mother refuse the offer... Leo even called Backalive a "creap". The next morning, Leo had to sneak out to spend more time with Marsu, and was guided to the marsupilami nest and family, but Backalive followed them and traps the wife and the Marsupilami Triplets, using one can of special gas to knock them out (littering with it as a dead giveaway to Leo), and another can to cover the tracks so Marsu can't smell his way to the rescue. Knowing it's a big risk, Leo inlists the help of his own parents, and with the help of the whole Newman family, Backlive was outmatched, Marsu reunites with his family, and Leo's friendship with Marsu was finally given David's seal of approval.
After that mishap, Leo had many adventures with the marsupilamis in the jungle, as Marsu becomes his best friend, and when they are together, there's always a ton of cool things to do, and never a dull moment.