Well, he’s basically showing his true colors.  You basically have to retrace his time on the show to see why it’s notable, and I saw a post earlier that covers this well, but the gist is that… *spoilers under the cut*…and this is bearing in mind that he’s with the Academia Fusion faction as we’ve come to find out–he went to LDS to spy on them (possibly having to do with Ruri being taken), then when Yuya comes along with his Pendulums, he gets more interested in them and tries to get close.  Because as Reiji’s scenes seem to show, Pendulums could be the tool that aids one of the sides in the summoning-type war that’s being set up (the details sorta escape me, but I think the idea is that Fusion’s the enemy, Xyz has it out for the Fusion guys over Ruri, and [given the recent news about Yugo] Synchros are sort of neutral).  So then he duels Shun, mostly toying with him (like he did with Yuya at first too), and when Shun forces him to get serious, he ups his game a bit, yet is still toying with him, and at this point, the whole Fusion/Xyz rivalry thing plays a role in how Sora belittles Shun (“You’re just prey for our hunting games!”).  Rewatching a lot of the plot episodes (like 18, 21, and 24) might help lol.

And so Shun beats him, yet Sora, out of a sense of pride maybe, insists that he would’ve won if he had gone all-out, that he wouldn’t let himself lose to “Xyz scum”, and Shun just walks away.  There’s more we need to see about him to try and connect things better (or at least I do lol), and we’ll probably get some more answers in the next episode or two when Sora challenges Yuto, but yeah, from what I make of it, basically the kid who was as sweet as candy turns out to have had a bitter side tucked away.

I wouldn’t be surprised if there was some memory issue going on ala Bruno, where dueling Shun brought up some hidden memories he had.