Kawashima as a head doesn't really work, but if they gave him two hands a la Andross, he would make an excellent fighter.
Kawashima's head would be about the size of DK, and would be about Link's weight. His hands would have permanent super armor, but hitting his hands while in the middle of an attack would cancel the attack.
His A attacks would revolve around his absurd range (he would essentially be a long-range melee fighter), and his smashes would involve headbutting or just plain smashing people with his head. To compensate for his range, his attacks would have lower-than-average knockback, requiring him to get closer to KO with his more powerful smashes. His aerials probably wouldn't be that great, but a cool Dair would be throwing his hands below him, grabbing someone, and falling on them.
Grabs would have excellent range (about the same as Samus' Extended Grapple) but low knockback as well. However, his grabs are fairly quick, and he is an excellent chainthrower against heavy targets.
B moves:
B: Equation - Kawashima throws his hands out to the left and right (same distance as his grab), and grabs up to two people. He then completes a random equation, and deals damage equal to the answer to both characters. If the answer is negative, it recovers them a bit. -5 to 30% damage, no knockback.
Side B: Stylus Swipe - Pulls out a stylus from nowhere and swings it in an arc in front of him. The stylus deals 10% damage with little knockback (like Mario's cape), and a black trail is left that you can't move through. It dissipates in about 5 seconds.
Down B: Dual Screens - Kawashima projects a transparent copy of himself in front of him. The copy can negate up to 15% damage, but if it absorbs more than 25% in one hit (ie. a Falcon Punch or Bowser Fsmash), it throws Kawashima into a broken shield animation. The attack has little prelag, but the copy has to fade away (which takes about half a second) before Kawashima can move again.
Up B: Microphone - Kawashima turns around and blows into an invisible microphone. This attack has about the same speed and distance as Melee Ness' PK Thunder, but Kawashima's head deals 10% damage with high priority to any direct attacks.
Alternate Recovery: Tether Grab - Kawashima throws his hands toward the nearest ledge. If his head is within a certain distance (about the same as Mewtwo's Teleport), the recovery is successful.
Final Smash: Your Brain Age - The characters are teleported to a Brain Age-esque screen. All characters excluding Kawashima must complete three multiple-choice questions in rapid succession (using the Control Pad to select answers). After the questions are done, Kawashima gives them their "Brain Age" (how much damage they were dealt), and normal play resumes. The FS deals 20% damage regardless, but each wrong answer adds another 20% to the total as well as 1 second of stun once they return to the battlefield.
See? It can be done.