Marcus Damon (Japanese name: Masaru Daimon) is the main protagonist in Digimon Data Squad. He is the only main character in a particular season to not wear goggles. His partner digimon is Agumon. He is also called "Boss" by Agumon. Marcus's rival is Kouki and his archenemy is Akihiro Kurata. He is also the love interest of Yoshino Fujieda.
Marcus's main personality is fighting and he gets cocky sometimes. To Digivolve his partner, Agumon, he must use his fist to punch an evil Digimon to gain data in his Digivice. Marcus is a fourteen-year-old student who loves to fight and, in his own way way, considers himself to be an "ultimate fighter".
Marcus lives with his mother, Sarah, and younger sister, Kristy. He meets and fights Agumon in the first episode of the series. When the fight ends in a draw, Agumon agrees to become his partner. Soon afterwards he's recruited as a member of DATS. As Marcus' partner, Agumon idolizes him and refers to him as his boss. Marcus is one of the few shown to fight Digimon by himself, though he still counts on Agumon's help to fight the stronger opponents. Even though others can evoke their DNA charges by themselves, Marcus must connect a punch on an enemy to do so, subsequently using it to digivolve Agumon.
Before leaving the human world to confront Merukimon, Marcus gained an alternate motive for going, which was to find his father Spencer, who he believes to still be alive, and bring him back home. However, the only clue he has about his whereabouts was given by Merukimon himself, who revealed to be a friend of Spencer and said he set off in search for King Drasil, the God of the Digital World, in his effort to build a co-existence between humans and Digimon. Marcus only learned other things about Spencer when he and his friends reached ElDradimon and learned that the people there have a debt of gratitude with him for helping the city years ago. They swore to protect the city from Kurata's army, but, in the end, there was little that they could do to prevent ElDradimon from being destroyed. Marcus caught Agumon after reverting to egg form.
Another moment of despair occurred when Thomas, for personal reasons, was forced to side with Kurata, and he had to fight him. During the battle between ShineGreymon and MirageGaogamon, Like Tai and Takato, Marcus' anger triggered the dark digisoul inside himself, dark digivolving his partner to a corrupt and rampaging version of ShineGreymon Burst Mode called "ShineGreymon Ruin Mode". After finally stopping him, he returned to a Digi-egg state, but it did not take long for Agumon to return to life and digivolve to ShineGreymon once more. Imbued by the determination to protect those that he loved dearly, his friends, his fellow DATS members who were close friends akin to family and, of course, his family themselves, Marcus was able to bring him properly to ShineGreymon Burst Mode to fight Belphemon. After Belphemon's defeat, Marcus Digivise the DATS team faced a threat even greater when King Drasil decides to wipe out the human world to end the struggle between the worlds once and for all, with the help of the Royal Knights. Marcus confronts King Drasil himself, just to discover that he is, in fact, Spencer Damon, his long lost father. After being rescued by Kentaurusmon, Marcus intends to go back to the Digital World to confront his father.
After meeting up with Gotsumon again, Marcus and Agumon faced off against Craniamon once more, where the Geogrey Sword actually broke. With help from the others, they were permitted to see King Drasil. Upon reaching him, Marcus fought his father, but couldn't land a hit. However, after hearing his father's disembodied voice, and with help from the Digimon, Marcus was able to connect a hit. BanchoLeomon appeared to help Marcus, only to reveal that he was holding the real Spencer inside his body before grabbing King Drasil and telling Marcus to kill them both. With severe hesitation, Marcus ordered ShineGreymon to unleash the attack that took them out. Just before the attack hits, Spencer thanks Marcus. With King Drasil seemingly defeated, and the World Tree destroyed, everything seems like it's over. However, King Drasil's true form appears, and it took a hit from Marcus to convince it not to continue and rest to see the results of a human and Digimon co-existence, reviving Spencer. In the series epilogue, Marcus decides to stay in the Digital World and help Agumon keep the riff raff in line and out of trouble.