Look at the class paths for each of the protagonists and recall their backgrounds, as well as their respective antagonists. Each of the paths ends up being a foil to the other!
Duran: His Light classes — Knight, Paladin, and Liege — are heavily based on his father, the Golden Knight Loki. The Liege class even changes Duran's armor to gold, making the parallel very obvious. Duran choosing the Light path shows him following in his father's footsteps. Liege, in particular, emphasizes healing magic over personal combat strength — by choosing that class, Duran would be putting the good of others ahead of his own desire for power and glory. In contrast, the Dark classes — Gladiator, Edelfrei, and Duelist — more closely reflect the Darkshine Knight, which is the corrupted, evil version his father was turned into post-resurrection. Duran's Dark classes involve him becoming obsessed with the idea of power, and are the only class of his that learn an arsenal of offensive, elemental spells. He insists on adequately performing magic and being a strong fighter, wanting to have the best of both worlds and reflects his original pursuit for his journey: getting strong enough to fight Koren on equal footing.
Angela: Her Light classes — Sorceress, Grand Diviner, and Archmage — focus on letting her natural talent and skills evolve on their own, embracing that she always had the power to use magic before. She merely never had enough of an incentive to cause them to evolve. The classes also lean towards wanting to emulate her mother, whom she looks up to. Her Dark classes — Mysticist, Rune Master, and Magus — take a path of power, wanting to pursue as much magical ability and strength as she can possibly get. It focuses less on natural ability, and more on forcing the magic to bend to her control. This path basically makes her similar to Koren.
Kevin: His Light classes — Monk, Divine Fist, and Warrior Monk — search for a balance between his human and wolf side. This is noticeable by his Light path being the only path to learn the Pressure Point skill, which allows Kevin to tap into the strength he has in his full-fledged wolf form, while remaining in his human form. He also learns healing magic on this path, perhaps wanting to prove that a beastman can do more than cause injuries, that they are also able to heal them. His Dark classes — Brawler, Enlightened, and Fatal Fist — embrace his wolf side, and likely writing himself off as no better than the non-hybrid wolfmen, whom he always saw as being needlessly bloodthirsty and violent. This path doesn't learn any skills, until the third class is picked. And those skills are Moon Saber, which saps the opponent's HP with every hit, and Energy Ball, which increases the rate of critical hits, wanting to cause as much pain and damage as possible.
Charlotte: Her Light classes — Priestess, High Cleric, and Sage — lean towards the 'good' side of heritage. Her initial class and the High Cleric class lean towards her late father, who was a Cleric himself, and towards her paternal grandfather, the Priest of Light. Her Sage class has her explore the elven side of her heritage. Her Dark classes — Enchantress, Necromaner, and Warlock — have an element of Corrupt the Cutie to them, and show how far her love for Heath can go. She is willing to walk down the same path as him, if it means she gets to continue to be with him. And it shows the 'bad' side of heritage, too. Both paths for her show the upside and downside to a person's family, and the influence they have.
Hawkeye: His Light classes — Ranger, Nomad, and Rogue — have a decidedly green and warm theme to them, making one think of a forest. Given that the Desert of Scorching Heat is said to have once been a large, beautiful forest, it certainly makes sense. This path recalls what the place used to be, and wanting to return to that time, before the Nevarl Thieves Guild was created. His Dark classes — Ninja, Ninja Master, and Nightblade — portray his thief side and continued association with the Thieves Guild, in general. It not only shows his parallels to Bill and Ben, with the Ninja Master having skills identical to theirs, but also shows the current state of the desert and guild.
Riesz: Her Light classes — Valkyrie, Vanadis, and Starlancer — reflect the motherly side of her personality. She cares deeply for her kingdom and her little brother, wanting to protect them from harm. This reflects in her skills for this path, too, which revolve around buffing her party. Her Dark classes — Rune Maiden, Dragon Master, and Fenrir Knight — show her embracing her role as the leader of the Amazon Army, and the warrior princess of Laurent. The skills for this path revolve around damaging her opponents and debuffing them, making it easier for her to cause damage. There's also a parallel to Bigieu in her Fenrir Knight class, who both use the Lamian Naga summon spell.
Angela's introduction has her walking around the Sub-Zero Snowfield in nothing but her typical leotard, so she ends up freezing and fainting from hypothermia. Yet every other instance of wandering around has no such problem. Then one recalls that she has since unlocked her magical potential. Perhaps she uses magic to not get affected by temperature.