Change your preferences here.
Whenever the holder attacks, it is healed by 1/8 of the damage dealt by the attack.
Shell Bell is a pretty poor item, not purely for the miserable amount of HP it heals per attack, but for how thoroughly it is outclassed by Leftovers. Leftovers will heal 1/16 of the holder's maximum HP per turn regardless of whether or not the holder attacked, making it ideal for defensive Pokemon—indeed, unless your Pokemon is doing over 50% damage to an opponent every turn (which is very unlikely, and in which case you would not really need Shell Bell anyway), Leftovers will be healing more overall, throughout the course of the match. Furthermore, most defensive Pokemon will not have the offensive ability to want to be attacking on every turn, rather than healing or supporting the team, and they are the ones most likely to want a healing item. So, as long as you have Leftovers, Shell Bell is not for you.
Driftveil City - Talk to old man in house (must have seen 50 Pokemon).