Guilmon is the partner Digimon of Takato Matsuda, the main human protagonist of the third anime series. His existence is unique, as he was not born in the Digital World but was created from the imagination of Takato, a young boy who dreamed of having his own Digimon. Takato designed Guilmon in his notebook, drawing a red, reptile-like creature and listing his attributes. After finding a mysterious Blue Card, Takato's device scanned these drawings, and the data was brought to life by digital entities known as the DigiGnomes, resulting in Guilmon hatching from a DigiEgg in the real world.

In terms of personality, Guilmon is characterized by a striking contrast between childlike innocence and fierce, primal power. As a newly born being, he is extremely curious and naive, possessing very little understanding of the world or the difference between humans and Digimon. Initially, he refers to his partner as Takatomon, thinking of him as another Digimon. He is normally playful, friendly, and trusting, which makes him very popular with Takato's human friends when they meet him. He has a notable and endearing obsession with bread, especially the loaves baked at Takato's family bakery. However, despite his gentle nature, Guilmon is a Virus-attribute Digimon. When his combat instincts take over, his personality shifts dramatically; his pupils contract, he begins to growl, and he displays a fierce, almost feral aggression that reflects his true nature as a powerful digital monster.

Guilmon's role in the story is central, as he is the primary Digimon partner to the lead character. He serves as the group's frontline fighter due to his exceptional strength for a Rookie-level Digimon, often battling enemies that would normally require a higher level to defeat. His existence drives much of the plot, as the other Tamers and the government organization Hypnos must deal with the consequences of a Digimon being born in the human world. Guilmon is also the physical manifestation of Takato's ideals and responsibilities as a Tamer; facing Guilmon's power forces the often timid Takato to grow braver and more decisive.

His key relationships are most strongly defined by his bond with Takato. Their connection is deeper than a typical partnership, as Takato is essentially his creator and feels a profound, almost parental sense of responsibility for him. They share a symbiotic relationship where their emotions directly influence each other, and they are capable of fusing through a process called Matrix Evolution. With his fellow partner Digimon, Guilmon is quick to befriend the easygoing Terriermon, and he enjoys the company of the mysterious Calumon. His initial relationship with Renamon is hostile, as her Tamer, Rika Nonaka, orders her to destroy the newly born Guilmon to absorb his data. Over time, this rivalry develops into a grudging respect and a strong allied partnership. He tends to either ignore or naively try to play with the mischievous Impmon, who often mocks him.

Guilmon undergoes significant development across the series. He begins as a blank slate, a powerful Digimon with the mind of a very young child who must be taught basic concepts. As he gains experience, he matures, learning to speak more properly and understanding his role as a protector. The most dramatic part of his development is his dark evolution. After the tragic death of the Digimon Leomon at the hands of the villainous Beelzemon, Takato is overwhelmed by rage and bloodlust, which forces Guilmon to dark-digivolve into Megidramon, a terrifying dragon whose very existence is a Digital Hazard that threatens to destroy both the Digital and Human Worlds. This represents the ultimate expression of his Virus attribute and the danger of uncontrolled power. Following this crisis, Takato and Guilmon achieve a true partnership by Matrix Evolving into Gallantmon, a noble, knight-like Mega-level form that represents the perfect balance of their human heart and Digimon strength.

Regarding abilities, even as a Rookie-level Digimon, Guilmon is exceptionally powerful, showing speed, agility, and endurance that allow him to hold his own against higher-level opponents. His signature techniques include Pyro Sphere, where he spits a concentrated ball of fire from his mouth, and Rock Breaker, a powerful slashing attack with his claws that can shatter stone. On his chest is a symbol known as the Digital Hazard, which signifies his potential for immense, world-ending power. Guilmon's primary ability is to Digivolve into more powerful forms to face greater threats. His Champion (Adult) form is the brutish and powerful Growlmon, his Ultimate (Perfect) form is the heavily mechanized WarGrowlmon, and his true Mega form is the hero Digimon Gallantmon, which he achieves through Biomerge Matrix Evolution with Takato. A final, even more powerful form, Gallantmon Crimson Mode, can be accessed by fusing with the sentient vehicle Grani.