Calling it now: His scumbag personality is all an act because he's against the marriage too. He's too free-spirited and wants to get Sagiri to break it off to help him save face. Everything about him just feels...forced. It's extremely rare to find a guy who forces his views on others to this level. They typically keep that kind of attitude on the way down low and try to be charming and attractive. He's not.
"But wait: If that's the case, why do this? It'd be SO much easier to just have him talk to Sagiri and they can agree to it."
Yeah, except Sagiri made it clear throughout the series (and even now) that she's all about duty and puts the mission first before her wants or needs. My suspicion is Shakuhito (name of the flashy guy) did his research about Sagiri and ran into that, knowing talking to her wouldn't work, hence this plan I've suspected.
Of course I could be way off the mark and he really is a Scumbag Steve. Still, the fact the entire series is pretty bad at making hateful characters (annoying characters don't count) also leads me to believe he's putting on an act.
just my ywo cents.