Alvin's arte list foreshadows a lot about his character. For starters, Alvin is the only character in the game who has no Spirit Artes at all, which foreshadows the fact that he's from Elympios. His arte list starts off with traditional Tales hero attacks like Tiger Blade and Demon Fang; but he gains artes like Dragon Swarm, which hint at his Anti-Hero and Chronic Backstabbing Disorder traits. Lastly, most of his artes are Fire-based, which counters Presa's water element, hinting at a connection.
Earlier on, his pool of Link Artes with other characters is much smaller than everyone else, which foreshadows his untrustworthy nature.
He also has the most dark-elemental or "dark"-themed physical artes out of the party.
The way Teepo acts, including its obsession with Milla's breasts, despite the fact that it's only a reflection of Elize's inner thoughts. Teepo acts that way because Elize views it as male, and thus unconsciously makes it act in a stereotypically male fashion.
Aside of Milla's bosom, Teepo is particularly fond of Jude, being the most frequent victim of Teepo's affectionate face-biting. Once you learn that Teepo expresses Elize's inner thoughts, you'll realize that this is Elize's way of expressingher feelings for Jude.
When Milla loses The Four early in the game, she's no different from any other human. It seems as if the only thing that separates the Lord of Spirits from a puny human is control of The Four. This is because she really isn't any different from other humans. The Four told her she's Maxwell and so she believes it.
In Fort Gandala, Rowen attempts to use an arte to unlock the door and break the hex chains on the prisoners legs. Yet he needs the others help - Jude, Alvin, Elize and Driselle. Yet with five people working together, they struggle to gather enough mana until Teepo wakes up. This is because Alvin is from Elympios, and has no mana lobe - and thus despite pretending, he cannot help with the spell at all.
And of course, the reason why Teepo waking up is so helpful is that Teepo is a booster, meaning it would be able to increase Elize's mana output and make enough to save the day.
Also even with Alvin not being able to produce mana, Jude, Elize, Rowen, and Driselle should have been able to stablize is it at least but still couldn't. Because Jude's dad is from Elympios, thus Jude has a smaller Mana Lobe meaning he isn't contributing as much mana as the others!
During a skit, the characters decide whether they're a cat person or dog person. Alvin says he's a cat person. Now consider that his ex-girlfriend styles her hair to look like cat ears, and has a swishy tail. His reaction to Rowen's comments about cat owners being too obsessive and mimicking cats becomes rather funny in hindsight.
Milla being an Instant Expert with swordplay is actually justified for one reason: muscle memory. Even if the Four Great Spirits were doing all the work guiding Milla's body, the fact remains that her body would register how it moved during practice, along with the sensations of how a sword would swing, the flow of combat, and all manner of similar aspects. The result being that Milla has two decades worth of mastery-level combat to draw upon, even if it's not her own, after getting over the stumbling block of having to actually do it herself.
Presa's story. Towards the end of the game you find out that she used to be in a relationship with Alvin and was a spy who formed relationships with high ranking military officers. Alvin betrayed her and she was caught (although it's possible he didn't know about the spying). She doesn't elaborate but says she went through some terrible experiences as a result of this. Considering how female spies, especially those who used sexuality, have often been treated, this gives rise to some nasty implications.
