Mister Mosquito

Note: This game is titled "Ka" in Japan.

In-game reset:
Hold L1 + L2 + R1 + R2 and press Start + Select during game play.

Kaneyo Mosquito:
Hold L1 and quickly press Up, Right, Left, Down, Square(2), R1(3) at the character selection screen. A shout will confirm correct code entry.
chkun.

Kenichi Mosquito:
Enable the "Kaneyo Mosquito" code. Then, hold L2 and quickly press Up, Right, Left, Down, Square(2), R2(3) at the character selection screen. A shout will confirm correct code entry.
chkun.

Two player duel mini-game:
Hold Start + Select on controller two after turning on the system. Press Square to move forward, X to jump, and Triangle (when the power meter is full) to do a rocket attack. Knock your opponent out of the ring to win.
chkun and HeavyMetalKid88.

Reckless Cyclist mini-game:
Rotate the Right Analog-stick thirty times at the title screen. Alternately, hold L2 and continuously rotate the Right Analog-stick clockwise at the title screen.
chkun and HeavyMetalKid88.

Year 2:
Fill each of the sixteen Ex Tanks to unlock Year 2.

Special costume:
Unlock all alternate costume colors on every level.

Glitch: No wings:
When you land on someone to suck their blood, look at the mini-screen in the top right corner. He will not have wings, while he does in the main screen. Also, the skin he is standing on usually does not look like the actual part of the body.
Zeotronic.