Heracles - Who doesn't know Heracles? Or Hercules, to those of you whom this mythology is still Greek to. And to confuse matters even more, you have to name him something else at the start of the game anyway. Just don't name him 'Hera', 'Ares' or 'Zeus' and you'll be just fine. In any case, this is the only playable character in the game, and unlike most RPG protagonists, Heracles learns no magic or special attacks, so he'll have to rely on his brute strength and the help of companions to succeed.
Ares - Ares is the hot-head god of war. ...I know what you're thinking, but trust me, this guy precedes Kratos by quite some time. Anyway, Ares, like Heracles, has no special abilities or magic, just brute strength. As the first god to join, he is quite useful early, and his stats are actually quite good, but his lack of versatility limits his usefulness later on.
Apollo - Here comes the sun god... Apollo that is. He's Artemis' twin and has the second-highest attack stat of any god. Plus, he has also mastered all three of the "Hyd-" spells. Not too shabby.
Artemis - Artemis' twin sister is the goddess of the hunt. She can use her Moon Bracelet to mute enemies, but more importantly, her regular attack strikes all enemies in a grouop at the same time. Plus, her leather bikini makes her a look a bit like Princess Leia in Return of the Jedi, and that's always a good thing, right?
Athena - Athena is the goddess of war. And the goddess of about a bazillion other things. Athena's the last god to join Heracles (by level), but she packs quite a wallop. Not only does she have the highest attack stat of any of the gods, but she automatically attacks twice in a row! And if that weren't enough, she can use her items to petrify or sedate a group of enemy without having to worry about MP. Whew!
Demeter - Demeter is the goddess of the harvest and the mother of Persephone. Like her daughter, Demeter knows healing magic, but Demeter is much more proficient at it. In fact, she's the best healer in the game, and she has tons of MP to boot. But her other stats, with the exception of Agility, aren't that fantastic, unfortunately.
Dionysus - Whoa, before you get started, I have something to say... This is just Bacchus. But Bacchus is his Roman name, see? This game's names are all based on the Greek equivalents... therefore, Dionysus he is. Dionysus has the highest HP of any god, and great defense, but his true strength are his different wines. They have various effects on your party and the enemies, so try 'em out, will ya?
Hades - Liek OMG SPOILERZ! The main antagonist of the original Glory of Heracles ends up joining the party after Heracles rescues him from his doppelganger. Hades isn't physically overpowering, but he does know the devastating "Buz-" spells, making him quite the potent offensive mage.
Hephaestus - Hephaestus is the gods' blacksmith. He can double or halve the enemies' or your defense, respectively, which isn't as useful as it sounds, and he has a Lava Staff to attack the enemies with, but his starts are quite underwhelming. His biggest use is his ability to remove rust from equipment without needing to use a Whetston or visit a blacksmith.
Hermes - In addition to being a bureaucrat and an Olympic level limbo champion, Hermes is the messenger of the gods. Oh, and he also got second-runner up in the Heracles look-alike contest! Hermes isn't quite as fast as you might expect, but he does have a sleep-inducing spell, along with warping spells. He also has the OddSack, which allows you to place extra items into storage, which is very useful until you get the MagicBag, which allows you to do the same thing, but without Hermes in your party.
Homer - Yeah, I know what you're saying, Homer's not a god, despite what die-hard Simpsons fans might want you to believe. But he does join Heracles for a short bit in the game, so bear with me. In this game, Homer's the founder of the town of Temis, and he'll give you his ship if you get rid of a boulder blocking its path and let him hitch a ride to Nana. As an assistant, he's not all that powerful, but he knows some decent magic spells and can attack a group of enemies for free with his Lion Staff, so he's not all bad.
Persephone - Persephone is the daughter of Demeter and the unwilling bride of Hades. Persephone is quite useful early in the game, as she's the first healer you can add to your party, but she gets outclassed by others later in the game.
Poseidon - You'll rescue the sea god, Poseidon, during the course of the game, and he'll join you after he rewards you with his ship/submarine hybrid. Poseidon has good stats, the useful Warp spell, and he can bring enemies' HP to critical levels with his Tidal Shield. And most importantly, he knows healing magic, which is ultra-useful. This alone makes him one of the most useful companions.
Triton - Triton is the son of Poseidon, and is apparently some precocious scamp and not a merman. Surprisingly, Triton has no magic or items, but he has blistering speed. His remarkably high Agility level means he almost always gets to attack before the enemy. But that's really all he has going for him.
Agos - Agos is apparently Dan's brother who lives in Reusis with his young son. Dan's habit of bragging has caused Homer to try to recruit Agos to help him explore the Desert Tower, even though Agos would much rather stay home with his son. I wonder who will end up accompanying Homer instead?
Aphrodite - Aphrodite is the goddess of love. Heracles rescued her from Hades in a previous game, and in this game she can fully restore any god or goddess' when you talk to her in Olympus.
Biff - When he's not terrorizing the McFlys, Biff is the resident blacksmith of Athens, and is supposedly a descendant of Hephaestus... But he's stopped working since he senses impending doom. And he just sits in his room sensing impending doom for the entire game. What a guy!
ChaosMage - The de facto lowest ranking titan guardian in Titans' Castle. In fact, there are several copies of this guy roaming around the castle as random battles, even though he's a boss. Pretty weak...
Cronus - Cronus is the most powerful of the titans, and the game's main antagonist. His cronies are scrambling to try to revive him, and Heracles is rushing to prevent that from happening. That's pretty much it.
Dan - Dan is the toughest soldier in all of Hebe. Apparently, the king won't talk to any outsider than cannot defeat Dan in battle.
Danos - Danos is in charge of processing all the barley that is grown in Camragi. This barley is needed to brew the Butterbur Beer that's popular in Loran. Unfortunately, Danos is so in love with Nina that he refuses to work until he's sure that she feels the same way about him.
Doppelganger - This creep is imitating Hades, and has usurped Hades' power in his tower. His true form is that of Orcus. Once you oust him, Hades will be rescued.
Doros - Doros is a wise sage, as well as the brother of the king of Menai. He lives in a tower northeast of Menai, and Doros was so enraged that his brother never sent him a wedding invitation when he got married, that he cursed the new queen, leaving her unable to speak. And he somehow turned into a monster during this time too. But once he sees how in love the two of them are, he turns human again and gives you the MgMirror. Makes sense, right?
Gigas - The most physically imposing of the four titan guardians in Titans' Castle. Luckily he has no magical abilities to speak of.
Hans - Hans is pretty much the perfect hero. Every good deed that has been done recently throughout the world was performed by Hans. He's gone off to save the queen of Nana, rescue shipwrecked sailors, halt pirates in their tracks, adopt orphans, and every other good deed you can think of, but where is he now?
Hera - Hera is Zeus' wife, but not Heracles' mother. How scandalous! While Hera is usually out to kill Heracles, in this game, she hangs out in Olympus and will tell him how much experience he needs to level up, and also tells him about the gods he can recruit.
Hestia - Hestia is the goddess of the hearth. She wins the prize for most useless immortal in this game. Pretty much all she does is sit around in Poseidon's shrine... and that's pretty much it.
Lord Vlad - His friends call him Brad, but to you, he's Lord Vlad. He's some big, vaguely vampirish guy who lives at the top of the Titans' Castle. Either finish him off and save the world, or Cronus gets fully revived... The outcome is up to you!
Kraken - Kraken hangs out at the top of Storm Tower and is the one responsible for turning Poseidon to stone. In retrospect, I'm more than a bit surprised they didn't give this role to Medusa... In any case, defeat Kraken and use the Trident to save Poseidon, will ya?
Nina - Nina is a girl who takes care of her little brother and grandpa in the boondocks southwest of Camragi. She is happy to share her never-ending supply of MedHerbs with travellers, and she is in love with Danos, even though she's too busy taking care of her brother and grandpa to let him know.
Queen of Menai - This young lady from Aros ended up marrying the king of Menai, and she became the queen. When the king forgot to send an invitation to his brother Doros, he cursed her so that she can no longer speak. Only the Herbs found near his tower will cure what ails her...
Queen of Nana - The queen of Nana is dead. Or sick. Or sleeping. Or something. In any case, things aren't quite right with her, and she exists as a bit of a microcosm to the chaos throughout the world that is being caused by Cronus. Well-loved by (almost) everyone, Homer and Hans are both doing all they can to try and save her from her sickness.
Tagon - Tagon is a mysterious mage that was hired by the pirates in the tower north of Privia. He turns out to be some sort of evil, fish monster working for Cronus. Once you defeat him, you can grab the Jail Key.
Tiamat - The dragonesque titan guardian found in Titans' Castle. Yup.
Typhon - Yet another one of the titan guardians in Titans' Castle...
Zeus - The big guy, Heracles' papa, Zeus. Zeus hangs out in Olympus, and aside from having a massive noggin, Zeus is the one who sent Heracles on his journey, and by talking to Zeus, you can switch which god you're traveling with, depending on your level.