Castor's younger sister. Together, the siblings form the Servant Dioscuri. Pollux respects her brother but acts as a reliable sister who admonishes him when necessary. While she is embarrassed by his blatant doting, she doesn't dislike it. She is friendly toward the Master and, perhaps because Castor fell to humanity, views humans as beings to be protected. On the other hand, the fact that her attitude barely changes from the time of summoning even as Bond Levels increase may be due to her nature as an immutable divine being.

Pollux's older brother. Together, the siblings form the Servant Dioscuri. Just like in the Lostbelt, Castor despises humans, often hurling insults immediately after being summoned. However, thanks to his sister's follow-ups, his arrogant behavior and sister-complex tendencies often miss the mark, making him somewhat likable. Furthermore, he possesses a cooperative side, listening to those he deems sincere and trustworthy, albeit reluctantly. As Bond Levels increase, he will bless the protagonist's journey despite his foul mouth. He maintains an aggressive attitude out of a singular desire to protect his sister from the humans on earth, but otherwise, he is a silent and powerful warrior. According to Jason in an Interlude, he already had this personality when they first met. Notably, when his sister is absent and he is alone, he becomes quite docile and speaks much more honestly than usual.

The default name for the protagonist in "Fate/Grand Order." The name was chosen by Kinoko Nasu when the TV anime adaptation was decided. In a later Takehouki Diary entry, it was revealed the concept was to have a name that felt natural for both genders. It is said that Fujimaru sounds like a masculine surname, while Ritsuka sounds like a feminine given name. While the kanji differ, the name is pronounced the same as Ritsuka Suse, another TYPE-MOON character. The name has been adopted for the protagonist in various manga and novelizations regardless of gender. Fan nicknames include "Gudao" and "Gudako," collectively "Gudas," or simply "Guda." In anime works, where a male protagonist is often chosen, he is frequently addressed by his surname as "Fujimaru(-kun/san)." In the "Fate/Grand Order Fujimaru Ritsuka Doesn't Get It" 2024 year-end special, Artoria Caster called the male protagonist "Ritsuka," marking the first time he was addressed by his first name in voiced media. As the sole remaining "Last Master of Humanity" at Chaldea, they throw themselves into an unprecedented quest for the Holy Grail alongside numerous Servants to prevent the collapse of human history. Due to the nature of the story involving multiple time-spaces, they have adventured in various eras and locations. In collaborations and other events, they are sometimes given fragments of information from other worldlines. They have experienced impossible levels of adventure in the Heian period, the Underworld, the Age of Gods, Yamatai, alternate worldlines, Imaginary Number Space, phantom continents, utopias, and ultimately Avalon, the Cretaceous period, the distant future, Heaven/Purgatory/Hell, and even Mars. They are Japanese, a citizen of Japan, and born in Tokyo. Their Spirit Origin alignment is Neutral Good. Their age is not explicitly stated, but they are certainly a minor from the start of the story (Part 1) through the Seventh Singularity. Regarding Part 2, the reason for refusing alcohol in Kiichi Hogen's Bond 4 voice line changes from being a minor to not knowing what the calendar year is (and thus not knowing if they are an adult) after clearing "Interstellar Mountain City Olympus," suggesting they should technically be close to adulthood by that point. On the other hand, in Constantine's Interlude (which requires clearing Traum), they are treated as a minor. In Summer Ibuki's Valentine scene, it is pointed out that they have stopped counting their age until Human Order is restored, so their age perception seems to be "provisionally a minor." According to Takashi Takeuchi, the visual motif is a gender-swapped Shirou Emiya and Rin Tohsaka. Because of this, they are often depicted as children or siblings of those two in fan works due to their similar appearance, but officially it is strictly a visual matter and there is no blood relation. As proof, the Pseudo-Servant group using those two as vessels (including him) and related parties never mention the resemblance. They have few lines, with their feelings only partially visible through dialogue choices, but they have a personality that speaks clearly regardless of who they are talking to. Holding Mash's hand until the very end amidst the flames, placing trust in anti-heroes generally considered dangerous, and even sharing food with enemies who attacked them—they are a kind-hearted person to the core. If they believe it is best for both the other party and themselves, they will actively put their body on the line. They feel fear and freeze up like anyone else in the face of terror, but they possess a positive spirit that never abandons hope. They generally don't dwell on negative things or sweat the small stuff, though it can't be denied they are a bit easygoing. They have a reckless streak, acting with an optimistic sense of danger—thinking, for example, that even a bomb might not want to explode every day—but they lack a heroic desire to save others at the cost of their own life. As repeatedly stated in the scenario that "he/she is an ordinary person," they have the weakness to give up relatively easily if they judge that "salvation is absolutely impossible." However, if there is even a shred of hope, they will cling to it, pushing through some recklessness to survive until the end and protect their precious junior. They are a protagonist who can only move forward with the help of those around them, possessing the strength that comes precisely from being an ordinary person. With a flat personality in a good way, they treat good people, villains, and even non-humans equally, never denying a Servant's past, ideology, or way of life. By maintaining a path, many Servants are influenced by their personality, becoming considerably mellow after being summoned to Chaldea even if they were originally villains.