I've always adored Erk. The reason is pretty superficial; he just happens to be this serious, smart and grumpy guy who is actually kind of awkward type of character, which have always appealed to me for some reason. But as I love him so much, I've actually spent a lot of time analyzing Erk's character, and I've found him to be much more interesting than he originally seems. This doesn't automatically make him a super interesting character but not even I expected him to be one, to be honest I expected much worse.
His admiration towards Pent is pretty adorable, and bases much of his motivation. He seeks to be a student worthy of someone like Pent, and this has led him to work really hard on his studies, to the point of passing out one time. Yet, despite how hard he works, he doesn't think he is good enough to be Pent's student. That is to say, he doesn't think very highly of himself; he has a low self-esteem. Erk is also a theory-based mage, so much that he tends to neglect other aspects of anima magic.
However, his real weakness comes in trying to connect with others. For a quick example, in his C support with Nino, she finds him reading a book and asks him to go back to a page which had a picture, but Erk tells her if she could leave him alone to read, and Nino leaves embarassed. Turns out Erk ends up cursing his lack of ability to talk to women after doing that, as he had no reason to tell Nino to leave. Louise also comments on how he always pushes away those who want to help them the most. Erk just answers he doesn't know how to answer to other people showing him affection or showing affection himself.
I also find him to be very underrated as a unit, but I already ranted enough about him.
So yeah, uh, Erk.
L'Arachel and Legault are also awesome, I'd specially love to just rant about Legault, too.