@zerosonico: The only problem is, they're literally selling you things that don't change the game at all. Sure, the upgrades let you go a little longer per game, but what do you miss out on when your games are short? Absolutely nothing. Me personally, I'm just trying to tell them to make things that sane people will actually buy.
The game oftentimes registers a correct answer even when my shape isn't turned the right way. Sometimes it'll even register two false correct answers in quick succession.
Just because I felt like it, I decided to jump right into the game on Hard mode. I figured that if it ever got too frustrating, I'd just go back down to Normal... and now, I shudder to think just how trivially easy this game would have been if I hadn't gone with the Hard difficulty. Honestly, this game needs to be a lot more challenging, or at least knock you back further than to the beginning of the current screen when you die.
This idea has been done many times before, contrary to the amazed reactions in other comments here. Not only that, but it's been done better. Those games had better piece lock sensitivity, pieces that you had to rotate to face the right away, better animations, etc. This one brings nothing new to the table.
Here's some ideas for what characters' unique abilities can be.
Kenji: Scrolls and Shadow Cloaks last twice as long.
Shiro: Initial speed is three times as fast as normal, acceleration is one-fourth the normal rate.
Sakura: Each enemy killed awards one coin, collecting coins increases Combo Meter.
Naomi: Can double-jump, automatically double-jumps whenever about to fall down a pit.
Orochi: While Combo Meter is at x100 or higher, Shadow Cloak remains in effect.
Daigo: While Combo Meter is at x100 or higher, Scrolls never expire.
The collision detection needs more work. At least three times now in 5 minutes of playing, I died without anything touching my square, and in several other instances survived overlapping other squares.
Combined with my suggestion that scroll upgrades should simply make them more frequent, not longer-lasting, I also think that if you have more than one scroll in effect at a time, their effects combine in special ways. In addition to their usual effects, if you have the following scroll combinations in effect... Lightning+Fire=Ignore Score Multiplier limit. Lightning+Gold=Get 1 Coin for each enemy killed. Lightning+Wind=Destroy all enemies on-screen with shuriken. Fire+Gold=Coins double in value.
Fire+Wind=Get x4 Chain and Score Multiplier per kill.
Gold+Wind=Coins increase your Chain and Score Multiplier by x1.
Zafnok, you can kill all enemies by just running into them. No sliding or ducking required. You can even kill those giant berserkers, who are normally invincible.
In Level 6 Challenge Mode, if any Centipede monster links to a Spider monster for even a split second, all Centipedes will get 5 Speed, and there is no way to reduce their speed. This effectively makes it impossible to get all 40 cores in Level 6; no matter how well you do in the preceding waves, the last wave will destroy you. (I conquered Challenge Mode on Level 21 with far less difficulty, so something's definitely off.)
If you make Ghost Hacker 3, you should add a new game mode where you solve programming puzzles, a bit like "Jahooma's LogicBox", only with a more tower-defense-y feel to it. In these puzzles you would build the towers and upgrades from basic elements and simple programs you created in previous levels. Ultimately, the solutions you give to these puzzles determines how efficient your towers and upgrades will be in the regular stages.
 "Super Stocktake"){kind=icon}
 "Ninja Slash"){kind=icon}
 "Live Puzzle"){kind=icon}
 "Mini Dash"){kind=icon}
 "XYZ"){kind=icon}
 "Shape Matcher Level Pack"){kind=icon}
 "Aqua Boy"){kind=icon}
 "This is not a minimalist game"){kind=icon}
 "Ghost Hacker 2"){kind=icon}
 "Stress Test Game"){kind=icon}
 "Pixel Pusher"){kind=icon}
You might try clicking the walkthrough button for that level on challenge mode if you haven't already. It should provide a valid solution.