You have to beat the Story Mode 8 times. Heihachi will then be unlocked.
The full unlock chart is:
Jin: Beat Story Mode 1 time
Violet/Lee: Beat Story Mode 2 times
Nina: Beat Story Mode 3 times
Lei: Beat Story Mode 4 times
Bryan: Beat Story Mode 5 times
Julia: Beat Story Mode 6 times
Kuma/Panda: Beat Story Mode 7 times
Heihachi: Beat Story Mode 8 times
Combot: Beat Story Mode 9 times