Tyranno Hassleberry, known in Japan as Tyranno Kenzan (ティラノ剣山, Tirano Kenzan), is a Ra Yellow Duelist who enters Duel Academy in the second year. To his surprise, he was assigned to the Ra Yellow dorm despite scoring high on the Duel Academy entry exam due to the fact that he did not attend an associated preparatory school. In both versions, people tend to call him only by his last name.
Tyranno made his first appearance when he complained to Vellian Crowler and Jean-Louis Bonaparte about not getting into Obelisk Blue. He then is seen heading a gang and stealing the Duel Disks of those he defeated. He was defeated by Jaden and returned the stolen items and came to idolize Jaden. (In the original version, this was supposed to be loosely based on the myth where legendary Samurai warlord Minamoto Yoshitsune defeated Saito Musashibo Benkei; Judai even compares him to the famed warrior-monk before the duel starts. Hassleberry's loyalty to Judai after losing to him, just as Benkei showed the same to Yoshitsune, also emphasizes this.) Although his dorm is technically Ra Yellow, he decides to live in Slifer Red to be closer to and learn from Jaden. From his introduction onward, he often fights with Syrus Truesdale over who is better suited to being Jaden's best friend. (In the Japanese version, it is over who has the right to call him "big brother").
In episode 73 it is revealed that Tyranno was hospitalized after a rock slide during an archaeological dig. The incident permanently damaged the tibia of his left leg but the bone fragment of a dinosaur fossil he unearthed during the excavation was used to replace it. Tyranno believed that as a result of Jaden's influence the dinosaur DNA awoke a great power within him which supposedly protected him from being converted by Sartorius into one of the members of the Society of Light. It is implied that this accident is also the reason for Tyranno's unique ability to narrow his pupils into reptile-like slits.
During the field trip to Domino City Tyranno teams up with Syrus in a Duel against the first duo of the Light Brigade, Frost and Thunder. Although the two are pinned down by their opponents, who tuned their Decks specifically to combat their cards, they end up pooling their efforts to combine their monsters into "Super Vehicroid - Rex Union". Despite their teamwork, however, Frost ends the Duel by wiping out all players but himself using Mirror of Duality and drags them to Sarina's lair from where they are transported to KaibaLand as bait for Jaden and Aster by order of Sarina.
At the end of the second year, Tyranno, along with Linda, chase after Prince Ojin as he prepares to arm the laser satellite. Unleashing the full force of his dinosaur DNA power and assuming a Duel Spirit form (appointing himself the title of "Spacesaurus"), he travels into space with "Elemental Hero Neos" to divert the satellite's blasts. While the returning Sarina keeps it from firing any further, the two are successful in destroying the satellite.
With the start of the third year, Tyranno chooses to remain in Ra Yellow instead of advancing to Obelisk Blue; to him, the issue of which dormitory he is officially a part of is irrelevant. Agreeing to aid Jim Crocodile Cook in his investigation of mysterious electromagnetic waves coming from the jungle, his dinosaur DNA reacts to additional signals sent by Thelonious Viper which render him uncontrollable. Jim Duels with Tyranno in a Survival Duel and successfully reverts him to his former self within the first few rounds of the battle but is beaten in the end.
When Jaden and co. go into the other dimension he is sacrificed when Jaden Duels against Brron, along with Alexis Rhodes, Chazz Princeton, and Atticus Rhodes. He is later discovered to be alive and held prisoner with everyone else who died in the Duel Spirit world in an alternate dimension by Yubel.
In the fourth season he pairs up with Blair Flannigan for the Pair Duels. He makes it to the final Duel against the pair of Jaden and Alexis during which Blair uses Partner Change so that she can be partners with Jaden due to her crush on him. Alexis later destroys the card and switches back to being with Jaden and defeats Blair and Tyranno. Comically, Tyranno displays fear of Alexis and Blair when they were his partner when they got angry.
In his Duel with Trueman, who was at the time masquerading as Daigo Sorano, his dinosaur DNA protected him from Trueman (who needed the opponent to carry some form of Darkness, which Tyranno did not have), until Trueman 'revealed' that the dinosaur bone in Tyranno's leg was that of a dinosaur he had eaten in his previous reincarnation as a dinosaur (in reality, Trueman lied to create darkness within Tyranno). Tyranno is then defeated but is revived when Jaden defeats Nightshroud.
Tyranno uses an Evolution Deck focused on Dinosaur-Type monsters. He uses cards such as "Big Evolution Pill" and "New Ultra Evolution" to quickly bring out his powerful monsters such as "Super Conductor Tyranno" and "Ultimate Tyranno". Tyranno also uses "Earthquake" in conjunction with his "Dark Tyranno" to attack his opponent directly. He also uses cards such as "Survival Instinct" or "Spacetime Transcendence" with his "Tyranno Infinity" to increase its ATK, giving him access to an easy to Summon, powerful monster.