- Note* This is just a theory, so if it is proven wrong, please don't spam me.
Ok, so I've had this theory in my head for a while now, and I think I have a enough evidence to show for it. My theory is that Luger is not the King's biological son, but Virgil's son (therefore making Trellis and Luger cousins). Lemme explain...
In book 4, ( I think?) we get to learn more about Trellis and Luger's backstory. We see that Luger heavily distrusts Virgil, and is openly disrespectful towards him. Most normal relatives would rightfully scold him for being so rude, but instead, Virgil just calmly replies with "How very sweet of you" or something along those lines. This could be a stretch, but it's almost like Virgil is trying to be nice to Luger. Later in the same book, there's a shot of Virgil's library and in the background, there's a photo of a female elf. Sure, this could just be a random background character that will never affect the plot, but there's still a possibility. Not only that, there is a lot more physical similarities between Virgil and Luger, because when we see the Elf King's backstory, the young King looks more like Trellis. From the lanky body type to the bushy bangs and semi- long hair, there's no doubt he and Trellis are father and son. Even more evidence can found with the amulets. We know that stones can be passed down between family members. For example, Vigo passed his stone down to his son, and Emily's great-grandpa passed his stone down to her. The Elf King couldn't have passed his stone down to either Luger or Trellis, because his corpse is being possessed by it. So that leaves their mother(s) to give them the stones.
Here's what I think happened: Virgil had a wife ( the woman in the picture) that died while giving birth to his son, and right before she died, she passed on her amulet to her newborn son, named Luger. Heartbroken over the loss of his wife, Virgil was in no proper state of mind to raise a child. So, he decided to give up his son to his then, not King of a brother, until he could fully healed from the loss. And maybe the king gave Virgil the false promise that he'll let Luger know who his real parents are. Years later, Virgil's brother has become the king, and has tasked him to take care of his "two sons". At first, Virgil is delighted. He finally gets to see his son again, but he quickly finds out that, not only is Luger completely unaware that Virgil is his real father, but he is taught to be untrusting and disrespectful towards him as well. Knowing that he can't reconnect with his real son, Virgil forms a bond with the young Trellis, and the rest is history.
This not only explains why Luger is not first in line for the throne, but it also explains the major age difference between him and Trellis. I know this is highly unlikely to be true, but it's still an interesting theory to think about.