This game is all an illusion, thought up by Marta.Marta is in a coma, and imagines this ENTIRE game. Not the original Tales of Symphonia. She played that game and loved it, so that's why the dream is based in that world. She didn't like Lloyd's voice, which is why he sounds completely different. Everyone that's an original character is from her actual life. Emil is the guy she has an obsessive stalker crush on. Richter is the most popular guy in school. Alice is the popular high school mean girl, and Decus is the good-looking loser she dates. Tenebrae is her dog. Brute is still her dad, and they've had a bad relationship thanks to adolescence and all that. Ratatosk Emil and the Vanguard are her bad thoughts and feelings surfacing.
Richter and Aster were more than friends.Let's face it, Richter and Emil act gayer than a barrel of monkeys the whole game. If Richter and Aster were lovers this makes a certain amount of sense, since Richter can't help but be affected by Emil's appearance even if he knows that he isn't really Aster. And perhaps on top of Emil's straight up hero worship his Aster-clone body has some vague spiritual memory of the original's attachment to Richter, even though Emil himself is in love with Marta.
Ratatosk took Aster's form to mess with Richter.Hey, it's not like Richter's going to attack someone identical to his best friend without a little hesitation first, and besides, Ratatosk is kind of a Jerkass like that. And given Richter's obvious discomfort around Emil the whole game, it was a pretty effective revenge. If you believe the above speculation, it just makes the whole situation that much more messed up.
The original cast have actually removed their Exspheres.This makes their limited power in this game make much more sense, and also avoids the oddness of everyone still using those things after the first game. As for the instances where Exspheres are mentioned, perhaps Sheena was lying so as to not worry Emil about whether or not she would be okay in battle (and she isn't; without her Exsphere she can't summon very easily), Regal is just so skilled with his hands that he can do hadokens anyway (and it's notable that this time he shattered ice. Not angelic metal), and Zelos and Colette have already developed their angel traits.
Ratatosk surpressed or quashed his kindness and conscience shortly after being betrayed by Mithos's groupEmil is said to be Ratatosk's kindness and conscience, not just an artificial personality. How did these aspects of his personality become its own separate entity, anyway? While it may also be due to how Ratatosk perceives humans (he thinks humans are weak, so he let his "weaker traits" surface), perhaps there's something else to it than that. Perhaps after Ratatosk gave Mithos's group the Derris Emblem and was betrayed by him in some unspecified manner, he began to harden his heart against humans and half-elves. The destruction of his tree just further angered him. Perhaps he felt that if weren't for his kindness towards both races and his conscience telling that it would be wrong, he could have wiped out both races before they killed his host tree. Thus he resolved to quash what he thought were his weaknesses. This side of his mind, long repressed was be allowed to surface when he awoke, but was too weak and suffering from amnesia. He might has also subconsciously knew that being aggressive towards people would not help him. Since his kindness and conscience was so diminished, and perhaps in a way "cut off" from the rest of his personality, these feeling which would become a separate personality lacking courage. Being harshly treated by the people of Luin didn't help matters.
Tenebrae is female
Aster and the real Emil Castagnier are related
The seal on Niflheim is going to break and becomes the source of Divine Powers.The breaking of the seal on Niflheim is confirmed by Tales of Phantasia where you even encounter the Demon King Pluto himself. Somehow Pluto managed to kill Ratatosk and when Mana dissapeared he granted people Divine healing powers.
Pluto stole the Eternal Sword from Claus(before he could seal it) and used it to go back in time to kill Ratatosk.The Demon King showed disdain for other races so why did he agree to give his powers to Claus without going all out in trying to kill the group(he stated he was merely testing their powers so he obviously wasn't trying to kill them)? Simple. He was planning on using Claus to get his Demonic hands on the Eternal Sword so that he could go back in time to the point where Ratatosk's seal broke(1000 years after Tales of Symphonia: Dawn of the New World since that's when it would be safest to approach Ratatosk) and killed Ratatosk and his Centurions just as Richter was freed from his duties(Pluto probably killed Richter too) breaking the seal at the point in time the seal was supposed to break.