We don't get to realize it until Book 7, but the Voice seems a little bit too desperate for Emily to kill Trellis. Then, Firelight comes along wire it's revealed that Trellis was one of the few individuals who discovered the truth about the Voice and was only kept alive because he was still useful, but was instead mind-wiped and brainwashed to keep him compliant. But the Voice didn't expect Emily to liberate him from Sybrian, he desperately tries to convince her to kill him so the truth about everything won't ever be revealed as a failsafe. That's why the Voice suddenly acts so cold towards her when she decided to spare Trellis, if she had chosen otherwise, he would've kept his end of the deal and help cure her mother.
Trellis saying that he just feels like he "has to" help Luger after the man, who had previously beaten him and left him for dead "for his own good", is reduced to a helpless old man without his stone. Even with the The Reveal about Luger later, it takes a trip to the fridge to realize that he probably felt that way because subconsciously he knew that Luger was his brother.
In The Last Council, its stated that the grouls are used to guard the most valuable treasures. This initially seems a bit odd, considering that there doesnt appear to be anything particularly of note in the catacombs Except for, say, the Mother Stone...
Two points from the fifth book, Prince Of The Elves:
At first glance the title seems to be referring to Trellis, since a major part of the story takes place in his memory. But the title can also refer to Max, whose history we also find out, and who was made into a false prince by the Elf King.
Secondly, at the start Max is seen sleeping in a tent with the flap open and in a decidedly unsuitable clothing for the snowy area he's in. Considering that he's essentially dead, with the Spirit's magic keeping him alive, no wonder something like the cold wouldn't bother him.
How come Trellis can't seem to defend himself properly in Firelight except at the end, when he was formerly so powerful before? To recall again, the power of the barrier is directly proportional to how focused and calm your mind is. When Emily was acting up, Trellis knew there was something wrong and is starting to panic, thus why he nearly got burnt when Emily suddenly attacked him.
It seems a tad bit overkill for the Voice to not only have Trellis be memory-wiped, but also enslaved for years, all the while making sure his people are mistreating him during so, whereas Luger was only wiped and still have a high position. Silas, Max and Emily didn't have a dark scout possessing them to do the Voice's bidding. This is is because unlike most of the other stonekeepers, Trellis was the hardest to manipulate, whether emotionally or mentally, because he had already learned the truth when he was a kid. The Voice sees him as an incredibly valuable asset, but, and contrary to most of the Elves opinions of Trellis being weak, he is still a big threat to his plans.
Thinking about it, when Gabilan tells Luger that he wiped his memories "by his father's orders," there was no real reason for Emily to make the jump to Luger being the son of the Elf King and, by extension, Trellis's brother as she did. Gabilan was a bounty hunter who could have been hired by any number of prominent elves. So there was no reason for her to think that "father" referred to Trellis' father as well.
Jos and Mat show that they're fans of Trellis by wearing wigs to imitate him, but the wigs are styled off Trellis' childhood haircut, which he'd, obviously, stopped wearing long before the story even started.
Later on in the series, Emily asserts that when they met, Trellis was possessed by Sybrian, a dark scout. A later book confirms this by showing when he was first possessed as a child. Since it's established that those possessed by dark scouts have no control over their own actions or even thought processes, and Trellis was specifically possessed to keep him from being a threat to his father, this begs the question of how or why Trellis tried to get Emily's help overthrowing his father during the time he was supposed to be possessed. Emily doesn't bring this up, either, despite being in a position to notice the contradiction.
Perhaps possession doesn't really equate to full-blown possession? Like when Joseph, a pilot like Navin, got possessed, he still retained some of his feelings that he tried to warn the others and claims he didn't want to kill them, before being forced to. Perhaps the shadows takes pre-existing feelings and warp them to the extreme? Like Trellis memories were wiped, but his feelings were likely still present, although he doesn't have an answer why he's feeling them, such as an irrational need to defy the King. So he tries to conjure a reason being jealous of Emily and desire to take the throne, when it's revealed to be actually one of morality.