Garen is not the hero.
Let that sink in a bit.
As most of you may know, Poppy is searching Demacia for centuries to find the hero only to be ignorant that she herself is the hero.
Now let's look at Garen. He's basically the paragon of what a Demacian soldier should be like. At least that's what he's been painted as. But as we all know, Garen has feelings for not just someone on the opposing side, but someone who is...well I don't think Katarina really matches any Demacian codes here.
Think about what this means to Garen. He's basically been your perfect soldier and an all around decent guy since he opted to have Taric exiled instead of executed. He has a reputation to uphold as the military poster boy. He's probably done all of this because he was raised to believe he wants it, but Katarina has ignited a flame of desire that he's unsure how to handle.
She's not just a noxian, but an assassin. A coward that avoids battle to slit the throats of men in their slumber. How could he fall for her? This question is probably what haunts him whenever she comes to his mind.
It made me consider something. Garen is not the hero not just because Poppy already is the hero, but because his future may not be with Demacia.
Something might happen that forces him to exile.
Just a thought.
Jarvan IV
The prince of Demacia that has been hardened by the wild. He combated dragons and many other vile beasts and, in that time, he found "enlightenment". However, the only thing that happened were his beliefs being strengthened. He now only believes in the glory of Demacia and sees all others as those against the crown. He will work without sleep to make sure his nation emerges victorious from this war, even if it means taking down those closest to him.
Garen
The never tiring, grand soldier of Demacia that represents justice at it's finest. His life has been devoted to Demacia and he sees only justice in what he does. However, as time goes on and he learns of Noxus and those within it, Garen will come to question his own ideals and those who command him. Is Demacia the shining light of hope he thought it was?
Poppy
Despite being a yordle, Poppy plays a big role in Demacia. While Garen and Jarvan are much more up front in their desire to protect their homeland, Poppy is never seen as someone who would be such a noble hero. However, as the war against Noxus continues, Poppy will begin to see how Demacia has changed since her late master gave her her hammer and others came to lead the nation. She will be a driving force in the future of Demacia, whether she likes it or not.
Xin Zhao
In Noxus, there is good and bad. Xin Zhao has seen nothing but the bad. Captured by the Noxus military and forced to participate in horrifying and gruesome events within Noxus, Xin Zhao knows nothing but hatred towards Noxus and will give all he can to make sure Demacia wins the war. As time passes and others begin to question whether what Demacia does is right, Xin Zhao will always be there for his king, Jarvan.
Lux
Much younger than Garen, Lux has served her nation for a long time. While I doubt she enjoys combat, she knows what is necessary to win a war. However, like her older brother, she will begin to wonder if what they are doing to the citizens of Noxus is right.
Quinn
While it might not seem like it, Quinn plays a major role in the fate of Demacia. As someone who can scout easily and knows how to infiltrate enemy lines, Quinn will learn things that everyone else does not know. She will learn of the good and bad of Noxus, and she will come to realize that, while the growing power within Noxus may be something that needs to be stopped, the brute force that is being caused by the Demacian army may not be necessary. Do the innocent citizens of Noxus need to suffer so much?
Fiora
Fiora is the ultimate wild card within Demacia. She likely represents the citizens of Demacia most. With very little current depth to her character, I will just let that previous statement sink in. Fiora represents the citizens of Demacia the most of most Demacian champions.
Vayne
Revenge. It is what fills Vayne's mind. The Black Rose is speculated to be the ones that have sent Vayne down such a road, and she will do anything to sate her thirst for revenge. As the Black Rose emerge, Vayne will also.
Kayle
After the Institute of War is completely done away with, it is likely we will see Kayle join Demacia since Morgana resides in Noxus. Her story will likely tie in closely to Morgana rather than the overall war, similar to Nasus and Renekton.
Swain
The Grand General of Noxus, Swain currently holds the highest position of power within Noxus and sees the world as his for the taking. He holds all the keys to every secret door within Noxus and his deep connections with the Black Rose have come to be a major tool in his rise to power. Swain has no morals and will do whatever it takes to take his position, even if it means doing away with those who would serve him to their death.
LeBlanc
The most known member of the Black Rose, LeBlanc is known for her deception and her ability to trick even the wisest of opponents. While not serving Swain to the end, without a doubt, she is a major piece of the chess board that is the power of Noxus. If Swain is considered the King, then LeBlanc is the Queen.
Sion
Swain's "blunt instrument" when it comes to dealing with things, Sion is "under Swain's command". His resurrected body likely holds little to no memories of his past life and, being fueled by a furnace of souls that is constantly refilled, he is truly an unstoppable force of nature. The day he is let loose upon Demacia is the day the war will end. However, how long will it be until Sion's need for souls begins to drive him to desperate and gruesome acts of terror. Such a powerful being is bound to lose control one day. Also, who knows if Sion remembers who he once was? If he does, does he agree with Swain? The Rook will eventually charge head on into this war and end it.
Darius
A truly potent soldier, he has become a true pawn of Swain. Darius believes that a world under the Noxian flag would be bliss and believes strongly in such. A world where those willing to show their strength rise to power to show their nation the path to travel down. As the war unfolds, Darius will show both sides what Noxus is truly about, whether that be through words or through combat. At the moment, however, Darius is nothing but the Pawn for Swain to use.
Draven
Every legend needs an anchor, and that is what Draven is to Darius. As the grand executioner, Draven gets to execute everyone who is anyone that come into Noxus through the war. However, Draven will begin to realize the brutality of Swain as every innocent soul brought into Noxus for death ends up at his feet to kill. He may have a huge ego, but everyone has a line to how brutal they can be. Draven knows his line. Draven knows when you cross from human to monster.
Katarina
Katarina's family of assassins know what Noxus is all about. However, that isn't what Kat's story is about. Kat's story lies in Demacia where Garen is. As their fates collide, they will both come to learn of the opposite side. They will both see that, perhaps, Demacia and Noxus may not need to be the enemies they now are.
Talon
Katarina's father trained Talon to be who he is today, and Talon respects him greatly for that. Talon could easily see Katarina's father as his own and is hot on the trails of learning what has happened to the man. As he searches for answers, he will without a doubt eventually come to the door of the one organization within Noxus that could possibly make the "King of Assassins" disappear: The Black Rose.
Vladimir
An ally of Swain and the Black Rose, Vladimir's gift of being an adept hemomancer has lead to him being a driving force into the necromancy side of Noxus. Being responsible for the resurrection of Sion, Vladimir's effects upon Noxus are slowly coming into view and he himself will slowly begin to effect the war in unimaginable ways. The Bishop will find it's way into the war and change it in ways that only a mad man could imagine.
Kled
Crazy? Yes. Unstable? Definitely. Evil? Absolutely not. Kled is a major force within Noxus. He is nothing less than a living legend. He knows how Noxus was back in the good old days and, while on the violent side of old Noxus, believes in the strong rising over all to maintain the future. He don't like what is happening these days in Noxus not one bit. Once Swain sees Kled as the threat he is, Kled will be forced to defend his land and make sure Noxus can defend itself from all the threats facing it. The Knight will have to take on the rising threat of Demacia and the rising evil within Noxus all at once, and he couldn't be happier about it.
Riven
While Riven has strong story elements within Ionia, she still has one purpose: to return Noxus to what it once was. She will ally herself with everyone she can to return Noxus to what it once was and revenge her fallen comrades. She believes in a pure Noxus, and the future of Noxus now is not that.
Singed
I seriously doubt the Mad Chemist will pass the chance of letting his brew be seen in combat during this war. Eventually, Singed will enter the fray and Riven will finally have her chance to avenge the fallen. Who will be victorious, however, is yet to be seen...
Mordekaiser
I BET YOU THOUGHT I FORGOT ABOUT THIS GUY! Yes, Swain isn't the only force of evil at work around here. As the Shadow Isles return, Mordekaiser seeks to reclaim his capital of terror: Noxus. As the war unfolds, Mordekaiser will eventually appear and, yes, he most definitely will. His path of destruction will leave both sides begging for mercy and it will take the strongest champions of Demacia and Noxus to stop this mace wielding tyrant from returning to power.
Aatrox
The Darkin of War, basically, will appear in this war without a doubt. As the war unfolds, he will be there to make sure it continues on... and on... and on... and on... until eventually, both sides have lost.
and 
Galio and Urgot
It is unclear how these two will interact with the war.
, they say."