Marco Diaz

Marco is kind, responsible, very organized, and supportive of others. He has a reputation at his school for being the "safe kid" due to avoiding danger and being cautious, though he insists that he is a "misunderstood bad boy" and lives for danger. He prefers keeping his possessions neatly organized. Marco is smart, gets good grades in his math and psychology classes, and figures out how to find a replacement for Star's wand charger using the metric system. Because he likes it when things go his way, he can become unstable when they don't, like when he snaps at Star for ruining his room and insanely trying to live in front of Stop & Slurp. Although he isn't used to the kind of craziness that Star brings, he admits that he would like a little danger in his life and finds fun in fighting monsters with karate.

He often struggles with self-confidence, afraid of being ridiculed by others, and can become nervous easily. "Naysaya" reveals that he has many insecurities and emotional hang-ups. He has an occasional temper and can get frustrated and/or annoyed with others, such as with Star when she intrudes upon his life, or recklessly drags him into overly dangerous situations. However, he soon makes amends with those who cause him grief, like thanking Pony Head for saving him in "Party With a Pony". He cares deeply for those he is close to, attempting to save Ferguson from being kidnapped in "School Spirit" and trying to cheer up Star whenever she is feeling upset. Marco also has an urge to occasionally protect Star and keep her safe, as depicted in "Sleep Spells" and "Blood Moon Ball".

Over the course of the series, Marco becomes less overly cautious and more confident, especially in his romantic pursuit of Jackie Lynn Thomas in "Freeze Day" and "Naysaya". He progressively takes more risks as a result of his friendship with Star, but he sometimes still struggles with self-confidence. After Marco moves into the Butterfly Castle, he usually tends to prioritize his wants over his responsibilities which can cause him to mess up his important duties such as him wanting a job higher up so that he could see Star or keeping Star's sleep-portaling a secret from Hekapoo so that they can continue to work together and go on adventures. Whenever he realizes this, he does his best to make things right.

In his role as Star's squire, Marco takes his responsibilities seriously and carries them out as best he can, as shown in "Trial by Squire", "Is Another Mystery", and "Divide". However, his intense focus sometimes causes him to ignore other people in his life, such as his parents in "Marco Jr."