He seems like the type that follows their dad's horrible orders to a T...but will still defy him if the need arises.
In the Treehouse footage, he says he really should have just killed Rinka & Suzukaze, but (sneakily?) defied their father's orders and relented because Camilla and Elise always get on his case when he does something to upset Kamui. Easily the coldest to Kamui out of all the siblings, but nicer than I thought. He also said Kamui teased him about his collar (lol), so maybe he does get along with Kamui to an extent.
I laughed when it cut to the three siblings while Marx and Kamui are fighting. It could basically be summed up as "Kamui, sweet child, don't get hurt" and "oh no, what do" while Leo(n)'s reaction was pretty much "damn it, why do I always have to deal with this shit" and gets the job done.
Overall, the whole scene gave him some points in my book.