Not much to say about Sirius honestly. He was pretty straight-forward in translating, though I will point out that he speaks informally, in a way that makes him sound both rough and young at the same time. For example, he uses もん (mon) instead of もの (mono) when referring to something that belongs to him. もの is also written as 物 to indicate a 'thing', though we don't see the kanji in Sirius' story.
I'm also not a huge fan of his view on girls/women which comes out in Ch. 4. He apparently views all women as weak and in need of protecting. I'm not saying our Princess here doesn't need protecting (she does, she's pretty bad at everything) but come on man, not ALL women are incompetent like our protagonist! Anyway, have fun reading!
Oh, one thing I forgot to add - Sirius does a lot of neck nuzzling. I mean, he's a dog-type prince, so I think he's supposed to be doing a scent marking kind of thing? I'm not totally sure, but it reminds me of the Alpha/Omega dynamics you find in fanfiction.