Guilmon is a curious and naive digimon, the youngest of the partner Digimon. He was created from Takato's imagination and brought to life by the Blue Card. At first, Guilmon had not been aware of the differences between humans and Digimon, calling his partner Tamer "Takatomon".

Guilmon is normally playful and friendly, proving to be quite popular with Takato's classmates for being a real live Digimon. Despite being portrayed as childlike, Guilmon does at times show deep insight. For example, when Takato didn't want Guilmon to digivolve, in fear that Guilmon would no longer be the same, Guilmon pointed out that Takato has changed over time as well, but he is still the same person.

Guilmon had one of the deepest realtionships with his tamer out of the whole series, the two did many things together and conquering many hurdles. Guilmon is very loyal and protective of Takato and would do anything for him. When Guilmon dark-digivolved into Megidramon it was because of their deep bond that Takato managed to come to his senses which allowed him to rescue Guilmon and bring him back, later allowing them to biomerge to Gallantmon to fight Beezlemon.

Source: Wikipedia