TL;DR: Ben Lupin and his hero Invy has flaws, making him not a Marty Stu.

Long Version: He has flaws. For example,he can't talk to girls without being embarrassing or blushing. Sure he may be perfect when he is Invy but Invy has flaws too. He clumsy at times like when he was spotted by a terrorist because of his cloaker emitting a loud sound because the cloaker drained in chapter 3 during the mission. How he turn invisible again? Well, he waited for the enemy for twenty seconds to regain his cloak back by hiding and he cloaked as the terrorist came up to him but he was not there.

Also, I would not judge a book by it's cover because Ben may be Marty Stu material at first but when you read beyond chapter 2, then you understand Ben/Invy's flaws. I can understand your own opinion and I will accept your opinion but overall, I would try to make Ben more of a good OC instead of a Marty Stu. Other than that, I'm sorry if I come off as a rant here, I won't do things like this for the rest of the story. Hope that helps explain.