Tetsuo is the most important character.
At the beginning of the story he is 15 years old. He is a friend of Kaneda, until the unknown power takes possession of him.
The power awakens after a crash with Takashi, a mutant with metaphysical energy. Tetsuo's power is forced by a special military acadamy, which experiments with small children in order to invent weappons on new basis. All capable kids get a number - so Tetsuo gets the latest 'No. 41'.
Because heīs not able to handle that power (which disturbes his character) he brings death and destruction. The more his power grows, the more people are killed.
Even his girlfriend Kaori is squashed by his monstrous body he gets, when the power within him becomes stronger than his mind (anime).
The only one with more power is Akira. Tetsuo awakes Akira. He is curious on the first place. He believes they would be stronger together. So they build the 'Great Tokyo Empire' which is finally ruled by Tetsuo's "Right Hand".
In the manga his transformation is less rapid than in the movie, and - in my opinion - more interesting. But in both stories you can see his schizophrenically character. Sometimes he remembers me of Dracula or Frankensteinīs monster.
Once you look at Tetsuos matrix you will notice his inferiority complex. He grew up in a childrenīs home, where he was kicked by the others. Even in Kaneda's gang he was the little boy. Kaneda was in the childrenīs home too. But Tetsuos character is not as strong as Kanedas. So he feels superior when heīs got the foreign power.
Tetsuo in manga
Tetsuo in anime