[@CleverUsername]

There are countless ways you could get on Herissmon's nerves. Deface his art, prod at him during one of his egotistical speeches, tell him there's a bug in his food...

But Reina? Probably startle her into thinking an attack is going on. Or try and poke at her during softer moments. That being said, [i]I[/i] wouldn't prank the human who's train of thought on enemy Digimon will eventually become "Alright, I'll punch it!"