The Fate/ franchise has been a force since Type-Moon's visual novel debuted back in 2004. It and its many, many spin-offs have been adapted into anime series, movies, video games, manga, and even stage plays. Every version of the franchise revolves around the idea of a "Holy Grail War" where mages do battle with each other by summoning heroic spirits from the past as familiars known as Servants.
Each iteration has been successful in its own right but the mobile game Fate/Grand Order has dwarfed its franchise brethren by an astonishing margin. That success has led to one of the game's chapters being developed into an anime series of its own, Fate/Grand Order: Absolute Demonic Front - Babylonia. That mouthful of a series throws a lot at new viewers so to help with that, here's a (relatively) spoiler-free guide to every servant appearing in the show's first season
13 Mash Kyrielight
Mash has been with Ritsuka Fujimaru since the very beginning of FGO's story and is right by his side once again as he delves into the mysteries of ancient Babylon. Despite her down to Earth demeanor and steadfast loyalty, Mash is an oddity, even among Servants.
She's a genetically engineered human being who was scientifically merged with the Heroic Spirit, Galahad, creating the first and only Demi-Servant. On top of that, she is the only Servant to manifest in the Shielder class giving her a unique defensive skillset. Those skills have been the biggest reason her master (and senpai) has made it this far.
12 Ishtar
Ishtar is the Mesopotamian goddess of beauty, war, and a whole host of other domains. As a Divine Spirit, she can't ordinarily be summoned as a servant so instead, she's merged herself with a human and become a Pseudo-Servant. That human should be very familiar to fans of the Fate/ series: Rin Tohsaka.
Because of this summoning method, the Ishtar of Babylonia is not the true Ishtar but is instead a fusion of her and Rin's personalities. She's an exceptionally powerful heroic spirit in the Archer class and has fully embraced Rin's tsundere nature, this time reflected in her attitudes towards the people of Babylon.
11 Enkidu/Kingu
Enkidu, a genderless Lancer-class heroic spirit, was shaped from clay by the gods and is famously the only true friend and companion of the God-King Gilgamesh (more on him later). Enkidu would have been a great ally for Ritsuka and Mash but he is quickly revealed to be a fake.
Instead, he is a spirit named Kingu who is only possessing the form of Enkidu and who serves the primordial goddess Tiamat. Kingu believes himself to be a prototype of what will replace humanity after his "mother" has wiped them all out. Despite not being the real Enkidu, he still possesses a connection to Gilgamesh that he himself doesn't understand.
10 Merlin
Merlin, the famous Magus of Flowers, is one of the most powerful Caster-class servants in the entire Fate/ franchise. He possesses the strength and skillset to preside over the rest of his class as the Grand Caster though the role is at the moment occupied by FGO's main antagonist, Solomon.
One of the reasons Fate's Merlin is so powerful is that he's not technically human. He's a half-human incubus which is also partially responsible for his scoundrel-like nature. Merlin possesses wisdom and intelligence befitting of mythology's most famous kingmaker, both of which are traits that are put to the test as he guides Ritsuka, Mash, and his angry young ward, Ana, through ancient Babylon.
9 Ana/Medusa
In the original Fate/Stay Night, Medusa served as the Rider servant but now she's back as a Lancer and in a very different form. Introduced originally as "Ana," the Medusa of Babylonia is summoned as the girl who had yet to be turned into a monster by Athena.
Ana is by far the most innocent form Medusa has ever taken in a Fate/ series and that puts her at odds with the decidedly more monstrous Gorgon. Ana spends much of the series struggling to balance her childlike innocence and affection for Ritsuka and Mash with the misanthropic tendencies ingrained in her legend.
8 Gilgamesh
Everyone's favorite golden Archer isn't quite as most Fate/ fans will remember him. Gilgamesh previously served as one of the main antagonists in the Fate/Stay Night visual novel and its prequel light novel Fate/Zero.
This Gilgamesh is far more likable and trustworthy and isn't technically even a servant at present but still qualifies as a Caster. He's even strong enough to summon other servants to aid in Babylon's defense against the demonic beasts. Until Chaldea's agents show up, he's the only thing holding the capitol city of Uruk together.
7 Ushiwakamaru
Better known as the name she took later in life, Minamoto no Yoshitsune, Ushiwakamaru is a Rider-class servant and an incredible swordsman summoned by King Gilgamesh.
Since she appears as her younger self, Ushiwakamaru tends to be a bit hasty and immature but remains a valiant ally and is by far the strongest offensive servant Gilgamesh has at his disposal. She quickly bonds with Ritsuka who is more than a little starstruck to meet one of his country's most famous heroes.
6 Musashibou Benkei
Benkei was a famous warrior monk who served under Minamoto no Yoshitsune and is considered one of the most powerful combatants in Japanese history. The only problem? This isn't the real Benkei.
This Lancer-class Heroic Spirit served alongside the real Benkei but at a critical moment his cowardice got the better of him and he abandoned his duties. He then spent the rest of his life assuming Benkei's identity and trying to atone for his sins by spreading Benkei's legend. That journey of atonement continues with his summoning in Babylon.
5 Leonidas I
The final servant summoned by Gilgamesh is the ancient king of Sparta of 300 fame. Despite being a Lancer, Leonidas legend at the Battle of Thermopylae has led him to manifest with primarily defensive abilities.
As such, he has taken up the position of the commander of Uruk's forces along the titular demonic front. His skills also lead him to feel a sort of kinship with Mash and he vows to train both her and Ritsuka alongside Uruk's soldiers.
4 Jaguar Warrior
Jaguar Warrior is one of the, um, strangest Servants to show up in Babylon. Like Ishtar, she's a Pseudo-Servant combing a Divine Spirit, the Mesoamerican Nagual Jaguar, with an original Fate/Stay Night character, Taiga Fujimura.
Jaguar Warrior seems like a joke on the surface, with her tiger onesie and speech made up almost entirely of cat-puns, but she does still wield the power of a (low grade) divine spirit so while she no match for the actual goddesses, she's paws-itively overpowering against an ordinary heroic spirit.
3 Gorgon
The other manifestation of Medusa's legend, Gorgon pulls exclusively from the monstrous stories about what Medusa became. She's an Avenger-class Servant and by far the most fearsome Heroic Spirit in the series.
She gains a significant power boost by not only using Medusa's legend but by channeling the Authority of the Primordial Goddess Tiamat. The mother of all demonic beasts is one of the show's biggest threats and responsible for more than a little anguish and heartbreak.
2 Quetzalcoatl
Unique, even among Babylon's many Divine Spirits, Quetzalcoatl is a fearsome, lucha-loving, Rider-class servant.
In the Fate/ universe, the deities of South America are able to transfer their existence and power from one human to another, hence the manifestation of this traditionally male goddess in a female form. She draws her power from her authority over war and the sun and uses it to take on humanity one wrestling match at a time.
1 Ereshkigal
The final Divine Spirit appearing in the series is yet another Lancer-class Servant. Ereshkigal is the Sumerian goddess of the underworld and the other side of Ishtar's coin. Though she is largely trapped in her underworld domain, she makes a few forays above ground via her connection with Ishtar's body.
Their connection is never fully explained, but it's safe to say that she and Ishtar are much more similar than either would like to admit with both of them even using Rin Tohsaka's form for their manifestation.