After being affected by the Virus Bullet, Amitie asks either Yuuno or Arf for help, but she points her guns at them while doing it. And after she wins the fight, caused by her own idiocy, she comically falls from the sky.
Vivio's first encounter with Yuuno in the storyline of Gears of Destiny.
After beating her, this exchange occurs (not a very accurate translation).
Vivio: No way! The Chief Librarian is strong! As expected of the "Ace of Aces'" magical teacher.
Yuuno: I told you I'm not a Chief Librarian! Wait, did you just say "Ace of Aces"?
Then a few moments after that...
Yuuno: Are you from another world? If so, where from and how did you end up here?
If you choose him against the Liese Twins, It's Personal for Lotte because she thinks a Dark Piece has copied her "beloved cute boy".
If you choose him against Stern, she fights him because he's Nanoha's magic teacher. After winning against her, she goes away while calling Yuuno "Master". Nanoha is confused that he's suddenly Stern's master.
Arf defeats Levi the Slasher in Sequence 3. As Levi went through the first game as a Chaotic EvilOmnicidal Maniac due to various reasons, Arf (and likely the players of the first game) expected the following to play out like your traditional "good trumps evil with the help of Unstoppable Rage" scene. Instead, she gets this: