Honestly, I was super excited when she got her power up. I didn't even know everyone hated her character till I looked at the thread after that chapter. I can understand most of your guys' points about her being a boring character and a leacher, but I see her as a character with a lot of potential. She was only leaching off Alan up to a point. Keep in mind that Alan has a very forceful personality and falling into step behind him was not completely her decision. She's also a mage, so I'm sure she was happy to have a party to travel with in the beginning, but at a certain point she probably realized that she didn't have much choice in the matter. Alan was one of the most powerful and famous players in the game, who wouldn't want to stick by him if they already had that kind of rapport built up? It's like: would you take a job that pays $100k a year but that you don't enjoy very much, or would you give up and hope to find something you like better (knowing that it probably won't pay as well)? Of course you could argue both ways, but if the $100k was just sitting in front of you, even begging you to take it, don't tell me you wouldn't be tempted. Also, the fact that she ended up being weak was not her fault. She was in an abusive relationship with Alan. Alan was forcing his followers to help him become stronger and trying to keep her at an equilibrium point where she was getting stronger but wasn't strong enough to head out on her own.

I was excited when she got her power up because it means that she now has the chance to become her own person. Given the situation and knowing Arc's character, she won't be traveling with him for too long. This is her time to become strong and develop as a character or to become resolved in her path of developing and becoming a strong character. I'm hopeful that it will be her who decides to split from Arc's party but I know that probably won't be the case. Either way, I think she's going to have some revelations in the near future and I think her character has a ton of potential built up.

As far as Arc and Roco, Roco is definitely the better girl right now. That being said, Arc made it abundantly clear that he thinks of Roco like a sister. There are a lot of great things that can happen now both good and bad. Now that this love triangle has been set up, we need to see how Lariette changes. On the one hand, she could be a diamond in the rough that brings out her true potential, and on the other hand she could end up having a shitty personality. I get the feeling that the author is pushing for her to be a diamond in the rough, but a lot of this comes down to his ability as a writer. If he continues to make Arc's character dull as fuck, then we're going to have a problem and the love triangle is going to fall apart and be super boring/infuriating. If he continues to force Lariette and Arc to come together without developing Lariette's personality and character, then we're also going to have a problem.

tldr: Lariette's character has a lot of potential that can be realized. I have faith that she will develop and grow as the story progresses, but it really all comes down to that.