- “You wear a helmet in the gym shower one time and they label you for life!”
- ―Marco's rant about his safe kid moniker
Marco Diaz is the deuteragonist of the Disney XD animated series, Star vs. the Forces of Evil. A Latino-American California citizen, he has been assigned the task of helping Star Butterfly adjust to life on Earth. He has also come to serve the additional roles of aiding Star in fighting all matter of inter-dimensional monsters, and apparently has even begun to form a friendship of sorts with her.
In the Pilot episode, Marco was chosen by his school's principal to guide and show Star around the school due to his reputation. As he discovered upon returning home on the same day (after having discovered Star's bizarre background at the end of the tour he had given her), his home had also been selected to be Star's own, though the person or persons who made this decision and the means by which it was reached remain as-yet unknown.
Immediately following his interactions with Star, Marco ran away from home, abhorrent of his new roommate. Star managed to find him afterwards, and acknowledged the hardship she had put him through. Prior to closure being fully reached, the two were ambushed by the villain Ludo and his thugs. After having helped to ward off the assassination attempt, Marco expressed a change of heart toward Star, and a preference for her to remain a resident of his home. The two headed back to Marco's house following this reconciliation.
Marco is practically the exact opposite of Star. He is very wary and tries to avoid any trouble or exposure. Because of Marco's cautiousness (and the one time he wore a helmet in the gym shower), he is labeled by both his peers and school staff as the "safe kid". Ironically, despite his predilection toward practical caution, Marco seems to enjoy the prospect of combat and adventure (In the pilot episode, he even expressed a desire for "some action and danger in (his) life". Marco also seems to want this to be opined so with his reputation, as well; Marco openly expresses disdain for being considered a "Safe Kid", and has repeatedly attempted (to little avail) to persuade others to view him as otherwise.
Marco has proven to be notably irritable, as seen in his rambling about his reputation in the pilot episode, and his very quickly losing patience with Star in the same episode. He has also demonstrated poor sportsmanship, elevating himself and slandering his opponents in the Arcade Dimension of the episode immediately following, though this may have simply been on account of his anger and defensiveness against Pony Head, due to her mistreatment and threats against him.
Marco seems to have studied martial arts extensively, namely Karate, as seen in the pilot episode, in which he employs it to combat the monsters who come to assassinate Star (even explaining it as such to her). He can also briefly be seen in the introductory animated sequence about to break a wooden board, a common test of strength and technique in East-Asian martial arts.
- Karate: Marco is learning and currently has a red belt in karate. He is very capable of using karate chops, kicks, and punches to battle monsters at the same level as Star.
- Culinary prowess: Marco is good at making nachos. In "Pizza Thing", he makes a pizza with Pony Head, which doesn't turn out especially good.
- Academics: In "Match Maker", it is shown that Marco has good math skills, getting good grades (including an A in psychology) and in "Quest Buy", using the metric system to find a charger for Star's wand at Quest Buy. However, he admits in "Naysaya" that he doesn't know how to do long division.
- Swordsmanship: As of "Running with Scissors", Marco has become proficient at wielding a one-handed sword.
- Survival skills: Sixteen years of experience hunting Hekapoo has turned Marco into a hardened survivor and warrior. Upon returning to his original age on Earth, it is unknown if he has kept these skills.
- Originally, in Daron Nefcy's version of Star without magical powers, Marco was named Sol, had an obsession with Dragon Ball Z (where his karate interest came from), and was more like an enemy to Star.
- In the midst of production, Marco was originally to have braces.
- In the unaired pilot, Marco's pants were originally blue instead of dark gray.
- Marco's middle name was revealed to be Ubaldo in "Match Maker".
- Marco's initials spell "MUD". This is purely coincidental, of course.
- Marco during Season One had a green belt with a white stripe in the Tang Soo Do discipline. A green belt represents "the growth of a seed that sprouts upward, becoming a plant. Like the sprout, the student is becoming something greater." A green belt is roughly middle level.
- Marco's fans are called "Meatballs" by Adam on his Twitter.