Hero XP

Do heroes gain XP, even when it says "0 XP" when you kill a monster?

I'm trying to level up all my heroes to LV20, and since Sousuke starts at 11...does this work?

Heros gain exp and levels like normal players do. However, if they are lower level than you are, they gain 5x the normal amount of exp.
So your lvl 11 sousuke will gain exp from any monsters from lvl 6 or higher.

For more exact information:
http://gw.gamewikis.org/wiki/Heroes

Scroll down to the part "Experience and Level".

how would people lvl their pets if this wouldn't work.
Remember to add all your low level heroes before accepting quest rewards in town. They will benefit from the Xp as well

But they don't get the multiplier from quest rewards.

Example: 3000exp reward. Hero won't get 15000exp if he's lower level than you.