Hmm, let me think.

All of them are multilingual and can speak every language that's spoken.

Len doesn't want to admit he's a shota. (he took one look at what a shota was and didn't want to admit he was one even though when he looked at Googled pictures of shotas he was there.)

Rin likes to tease Len a lot; she especially like to tease him in a sexual manner cause it makes him blush a lot.

Oliver and Fukase are secretly dating but no one knows about it. They're show that they're dating but everyone thinks they're just friends.

(These are just some I haven't listened to vocaloid in a long time)