1-Up: Rin (in a bin) gives you one of these if you hit her at the right time, though you can't carry more than one spare life.
Artistic License - Physics: Naturally. Possibly the oddest thing is that in Multiball, Hisao's clones somehow retain exactly the same horizontal velocity, even though they aren't connected to each other in any obvious way.
Epic Fail: The game has a few achievements for this, such as flying straight into a game-ending character right after using up your spare life, firing a perfect initial shot only to immediately crash into the game-ending character, or ending a run by grinding to a halt.
Expendable Clone: After colliding with Gendo twice to produce the necessary clones, the next special activates Multiball: two extra Hisaos enter the game. During Multiball, Hisaos can die, and the game returns to normal mode (in which Hisao cannot die) once only one Hisao is still alive.
Incendiary Exponent: Hitting Lilly, Rin, or Emi 3 times in a row sets Hisao on fire. His fireball will sometimes make a small explosion that gives him a speed boost.
Jekyll & Hyde: Gendo and Kenji are implied to be this (Gendo has Kenji's scarf for no apparent reason, and they cannot appear on the screen at the same time).
Joke Character: Rika, who has a "Super Boost" attack that only activates if you have enough Katawa Points. Katawa Points do not exist. They're a reference to an April Fool's joke from the Katawa Shoujo blog.
Jump Scare: Wait too long to click in Chie's game, and you'll be rewarded with one of these.
Luck-Based Mission: The characters are mostly arranged regularly, but this doesn't matter much once you get going decently fast and dozens of them fly past you every second. Chie plays this very straight with a roulette-based attack (and an achievement for getting a certain random result from it).
Non-Standard Game Over: Almost all games end with "Student Council Get," caused by crashing into Shizune with neither her special nor her Rumba mode active. But the game also ends if Hisao rolls to a stop without hitting Shizune, which earns you an achievement.
Power Nullifier: After you hit Hanako, she will block the effect of the next character you hit, unless Misha intervenes.
Refuge in Audacity: To a perhaps even more extreme extent than its predecessor; not only can you inflict countless acts of extreme violence on a flying arrhythmia patient (who can suffer a potentially fatal heart attack if he so much as bumps into someone), you can also needlessly shock him with a defibrillator to hurl him towards further acts of violence.
Self-Deprecation: The player guide immediately makes fun of itself ("Help files? Nobody reads help files!") and doesn't hold back on the game either: "If you don't click the mouse button at all, you may be rewarded with not getting hooked on this dumbass game."