At first, Guilmon doesn't seem very intelligent, not speaking much and calling Takato "Takatomon". However, in later episodes before the Devas arrive he is shown to become more intelligent. When Takato is afraid that his personality will change if he digivolves, Guilmon tells him he'll remain the same Digimon on the inside.
Guilmon is never one to back out of a fight, and seems attracted to Renamon, as whenever he senses her presence (or possibly notices her scent, as he does in Blame it on Ryo) he rushes over to her.
Guilmon also has a big appetite, especially for bread.
Introduction to Earth
Guilmon was created when Takato drew a picture of him and designed all the information on him. The picture and other papers were then slid through his digivice by DigiGnomes, enfusing the information with data to create Guilmon inside a Digiegg, which hatched and bio-emerged after Renamon killed Lynxmon. Takato tracked Guilmon down with his digivice, and initially brought him home while hiding him in a box (his mother didn't allow pets in their house, although his father was more sympathetic). Guilmon would often call Takato "Takatomon".
Eventually Takato hid Guilmon in the alley while giving him some bread. Guilmon loved eating bread more than anything, but when Takato went to school, Guilmon followed him there. When the principal, Mr. Kurosawa, saw him, he pulled the fire alarm, thinking him to be a monster. Takato eventually found Guilmon, but as he was finding a hiding place for him in a cave area of the park that no one ever goes to, Renamon attacked, and Rika told her to destroy Guilmon and absorb his data. However he was immune to her Diamond Storm; furthermore, Renamon didn't want to destroy a Digimon that was not evil, but had to continue the fight until Henry and Terriermon broke up the fight.
Later, Guilmon tracked down Renamon in an underground parking lot and challenged her to a rematch. The fight was broken up when Terriermon digivolved to Gargomon and lost his mind. Guilmon stopped him from attacking Rika.
Eventually Takato and Guilmon began to get along better with Rika and Renamon. When a digital zone appeared in a tunnel under a bridge, it caused Guilmon to disappear. However he was not destroyed, but trapped inside the digital zone. Takato was able to get him out before it was destroyed by Yamaki.
Impmon took Guilmon to terrorize people with fireballs, but Guilmon didn't want to do this and walked away. However, when he told Takato, he worded it to sound like he was the one who scared the people, and Takato began to trust Guilmon less. Shortly after he fought Devidramon, but was at a disadvantage. Then Henry and Terriermon told Takato that Guilmon was innocent, and Takato got over his trust issues with him, enabling him to digivolve to Growlmon and destroy Devidramon.
Dedigivolving or hiding Growlmon proved to be a challenge, but eventually he dedigivolved when a rainbow appeared.
Guilmon participated in the fight against IceDevimon, who froze him and Terriermon, but Henry unfroze them by inflating Terriermon. Guilmon later used a Speed and Hyper Wing combo to gain the upper hand on IceDevimon and destroy him.
Guilmon fought Musyamon, but eventually Terriermon digivolved to Gargomon (retaining his mind) and destroyed him. Later, Takato introduced Kazu, Kenta, and Jeri to Guilmon. Kazu and Kenta only got as far as seeing his eyes and thought he was a trick set up by Takato; Jeri thought he was cute.
Enter the Devas
When Mihiramon appeared, Guilmon was the third to fight him, digivolving to Growlmon and hitting him with Dragon Slash, but Mihiramon began to kill Growlmon with his teeth, until Takato was able to matrix digivolve him into WarGrowlmon, who destroyed Mihiramon.
While playing with Takato, Kazu, Kenta, Jeri, and other students from school, Guilmon later digivolved to Growlmon to fight and destroy Sandiramon, powered up by a card Kazu gave Takato. Kazu and Kenta helped hide him from Miss Asaji when he went with them on a field trip, and he was the only one who enjoyed Kenta's singing (they share the same voice actor). He also destroyed Sinduramon at the dam.
He did not participate in the fight with Pajiramon, as the tamers were going on a trip in which they could not bring him, but he fought against Vajramon when he returned, but was defeated alongside Gargomon, leaving Kyubimon to fight and eventually destroy Vajramon when she digivolved to Taomon.
Battle of Adventurers
Guilmon traveled with Takato on a vacation to an island with his cousin Kai. Takato hid Guilmon inside a suitcase, but collapsed trying to haul him to the house. Kai found him and became friends with him, while his grandfather mistook him for a talking dog. He battled and destroyed many of Mephistomon's minions, including Tylomon, Scorpiomon, Mantaraymon, Archelomon, Ponchomon, Pipismon, Sepikmon, Baromon, and, eventually, Mephistomon himself, using Trinity Burst with Rapidmon and Taomon.
The Devas, continued
Guilmon fought Indramon as Growlmon, but it wasn't until Takato used a blue card drawn by Kazu to digivolve him into WarGrowlmon that he destroyed Indramon. He also fought Vikaralamon as WarGrowlmon, but became trapped in a Fusion Ball. Takato used all his strength enabling WarGrowlmon to break out, throw Vikaralamon to the ground, and destroy him.
When Makuramon kidnapped Calumon, it was Guilmon who found the portal to the digital world, located inside a tunnel he dug in his cave. Afterward, Takato introduced Guilmon to his parents. His mother initially thought he was a monster after Vikaralamon's attack, but his father thought quite differently and promised to give them "Guilmon bread" when they returned. Guilmon traveled to the digital world with Takato, Henry, Terriermon, Rika, Renamon, Jeri, Leomon, Kazu, and Kenta to rescue Calumon.
When Rika, Renamon, Kazu, and Kenta were separated from the group by a data stream, the others searched for them and fought the motorcycle Behemoth to protect the Forgotten Village. Guilmon jumped on Behemoth and became possessed to ride it, but Leomon knocked him off with his sword, releasing him from Behemoth's control.
While returning to the flag to look for Rika and the others, Guilmon noticed Renamon's scent and they followed it, but lost it at a point where the four were taken by another data stream, and entered an elevator taking them to a warped area where Ryo and Cyberdramon reunited them with Kazu and Kenta. Upon return to the desert they fought Majiramon, and when Cyberdramon destroyed him, he left with Ryo to find another opponent.
Shortly after Rika and Renamon reunited with them, Caturamon sent Takato, Henry, and Terriermon into another data stream, separating Takato from Guilmon. While searching for the three, they fell into the forest area where they fought Orochimon to protect the Gekomon he enslaved.
Somehow Guilmon digivolved to Growlmon without Takato, while Kenta wanted to be Guilmon's tamer, especially after Kazu became Guardromon's tamer. Eventually they entered a data stream controlled by Caturamon to take them to Takato, Henry, and Rapidmon in the Land of the Sovereign, where they were fighting Beelzemon. When Beelzemon destroyed Leomon, Takato was sent into a rage and digivolved Guilmon to an evil Mega form, Megidramon.
Megidramon nearly killed Beelzemon, but after destroying Makuramon and absorbing his data, along with Rapidmon and Taomon's Ultimate data, Beelzemon overpowered him. Takato returned Guilmon to his senses and biomerged with him into his true form, Gallantmon, who gained the upper hand on Beelzemon, destroying Behemoth and Caturamon. Before he could kill Beelzemon, Jeri told him not to, after which Gallantmon separated into Takato and Guilmon.
WarGrowlmon fought Zhuqiaomon alongside Rapidmon and Taomon, and Zhuqiaomon nearly killed him as Guilmon until Azulongmon tried to break up the fight. Gallantmon briefly fought Zhuqiaomon before Azulongmon convinced him to end his vendetta against the humans and instead focus on the true enemy, the D-Reaper.
When the tamers were returning to Earth on an ark, Guilmon told the ark to stop when Takato wasn't on yet, and it listened to him and stopped. After arrival on Earth, Guilmon fought D-Reaper agents, unable to biomerge on Earth until Dobermon gave them this power. After destroying the D-Reaper agent Creep Hands, Gallantmon was pulled into the D-Reaper's chaos, separating into Takato and Guilmon who were attacked by Evil Jeri until MarineAngemon fought her off.
Eventually Gallantmon gained a noble steed, in the form of Grani, the ark. They fought the D-Reaper until it was clear nothing could be done to destroy it at the moment, forcing them to retreat for a week until Yamaki and the Monster Makers came up with a plan to defeat it. With a red card enabling them to retain Mega form inside the chaos, Gallantmon fought against the D-Reaper, transforming into Crimson Mode with wings due to Grani, and destroying many Horn Strikers and Evil Jeri, but after the D-Reaper was killed, Gallantmon separated due to a miscalculation by Shibumi. Guilmon threw Takato at Jeri with his tail, and before they could all be sucked into the void of Juggernaut, Kazu and Kenta arrived with MarineAngemon who saved them.
Due to the side effects of Juggernaut, Guilmon dedigivolved to his In Training form, Gigimon, and was forced to return to the digital world to survive. Takato eventually returned Guilmon and the others to Earth.
Guilmon fought against Locomon, and destroyed Parasimon's brother possessing Rika, and as Gallantmon he and Takato destroyed more Parasimon, and in Crimson Mode destroyed the main Parasimon and their digital zone. Guilmon was present at Rika's birthday party, with Impmon stealing food from him.
Pyro Sphere - Shoots a fireball from his mouth
Guilmon also seems to have a powerful tail as he uses it to splash Kazu, Kenta, and Jeri, and throws Takato toward Jeri during the final battle.
- Guilmon destroys exactly half of the Devas.
- Guilmon as the glutton of Digimon Tamers gives him resemblance to Goku from Dragonball Z, Serena from Sailor Moon, and Pinkie Pie from My Little Pony: Friendship is Magic.