Do Leo and Usagi argue, and if so, what would be their worse argument?
Leonardo: You can't fight this like we can! I know we're equals, but your abilities aren't enough to fight this. We can barely fight this!
Usagi: I will not leave you to fight this alone. I will do nothing but fight my best beside you.
Leonardo: I don't want you beside me for this. I don't want to risk your life if I fail. This isn't your fight.
Usagi: You are my fight, Leo.
No spoilers ;) but Leo and Usagi don't fight often. They have conversations instead. The fights they do get into is often Leo's self-sacrificial nature and Usagi's code of honor to fight to the death. They would both die for the other, and this fact upsets them both.
At the end of the day, they always know how to make up, though.