Marco is a silly teenager. He is often referred to as the joker of the group, and coined the name Animorphs, a portmanteau of "animal morphers". Marco's sense of strategy proves essential to the Animorphs' missions, although his methods can be somewhat ruthless, tending towards philosophy of the end justifying the means. He is slightly conceited about his good looks, dubbing himself the "cute Animorph", but he is self-conscious about his height. He and Jake have been best friends since they were babies.
Marco's dad is Peter, who lost interest in life for two years after the supposed death of Marco's mother, Eva, a Latino immigrant to United States. In reality, she was infested by Visser One roughly a year and a half prior to the series, and her death in a boating accident was faked as a way to get her off the planet. Throughout most of the series, Marco battles with himself about her fate, debating whether he should he kill her or try to set her free. He comes very close to killing her in several books, until he finally frees her in The Revelation. Marco's father was uninformed about the true whereabouts and status of his wife until that same book, when he is finally told the truth and is the first of the Animorphs' relatives to join the war effort.
Marco is reluctant to fight because he knows that his father would be devastated by his death. He also argues that they are simply kids and that this is not their battle. When he coins the phrase "Animorphs", he describes the group as "idiot teenagers with a death wish". However, he also has a strong sense of loyalty, and says that he will fight to free Jake's brother, Tom. He announces that he is quitting in #5, but when he sees his mother is a Controller, he vows to continue on fighting until she is free.
Like Rachel, Marco has a bit of a dark side, which shows up in his (at times) very cold and ruthless schemes. Marco is often focused on the "big picture" (winning the war), sometimes at the cost of the "little" things (his mother, for example). He isn't entirely comfortable with this aspect of himself, but has embraced it as necessary to the war effort. He also struggles between the dream of his mother returning to normalcy, and moving on, accepting her for dead. This conflict is revealed in The Proposal. In The Android, he develops a lust for gruesome revenge against the Yeerks, and admits to enjoying being violent (for example, he worries Ax when he tries to kill a beetle in spider morph), even though he constantly derides Rachel's violent habits.
Marco prides himself on an ability to see the clear path from Point A to Point B - to chart the most effective plan of action, with morals or values being secondary concerns. This is not to say he is amoral, as he is able to factor in these values and alter the plan accordingly - but he does have the ability to see what needs to be done to achieve goals. In this way, he is extremely valuable strategically.
At the end of the series, Marco has a flourishing post-war life; he has become a celebrity, starring in TV shows, giving interviews, and becoming the unofficial spokesperson for the Animorphs. He enjoys the playboy lifestyle, although he admits to being "bored out of his mind" and does not hesitate to give it all up and join Jake in a possibly suicidal mission to rescue Ax. He is the final narrator of the series.
Marco is also the only Animorph in the series without a main love interest, though not for lack of trying.
Jake: Jake and Marco have been friends since their childhood. Though their friendship is tested at times, they generally retain a strong bond. Towards the end of the series this begins to deteriorate, as Jake sinks into depression and Marco regains his family. After the war, Marco and Jake seem to be no longer close, though Marco is concerned for Jake and attempts to reach out to him. Even then there are several instances in the last book where Marco takes Jake's side - during the war especially. However, when they leave to rescue Ax from The One, it appears that their old friendship has been restored.
Tobias: At the beginning of the series the two aren't close, with Marco hassling Tobias occasionally, but eventually a strong camaraderie between them develops.
Rachel: Rachel and Marco's relationship is characterized by their playful insults and banter. However, they also support each other in times of need, such as when Rachel visits Marco at the end of #30. They generally side together on group votes, as both are more interested in pragmatism than ethics. They dated in the alternate universe of Megamorphs #4.
Cassie: Cassie describes Marco as cute, but not her type. The two don't directly interact often in the series. Though they are generally on good terms, Marco is often impatient with Cassie's moralism.
Aximili-Esgarrouth-Isthill: At first Marco dislikes and distrusts Ax, but as time goes on the two develop a close relationship. After Marco's cover is blown and he has to leave his regular life, he goes to live with Ax at his scoop, and the two bond over electronics and soap operas.
Including unusable and unused morphs, Marco has utilized 52 different morphs (the most of all Animorphs) in the series though it is implied he has obtained many more. This is ironic in that originally, Marco was the least willing of the Animorphs to be drawn into the war, yet he has acquired the greatest number of morphs overall.
Marco is one of the characters whose battle morph is often forgotten for merchandising and illustration purposes; although he very rarely uses anything besides his gorilla morph in regular battle, he was illustrated morphing into a wolf on the cover of Megamorphs #1, and a cobra in Megamorphs #4, while the other characters were all shown morphing into their preferred battle morphs. In the Animorphs TV series, he was usually portrayed morphing into a wolf (Cassie's usual battle morph), rather than a gorilla.
In 1999, four different Transformers toys of Marco were made. Two of the larger ones that were sold in stores turned into a Gorilla and a Fire Beetle, while the third was one piece of the Tyrannosaurus rex made up of Jake, Cassie, and Marco figures. The smaller transformer was part of a Taco Bell children's meal promotion and turned into a hammer head shark.
