An offensive hand grenade produces mostly blast, a defensive grenade produces blast and fragments. Defensive grenades may have a greater danger radius than they can be thrown, so they are best used from trenches. Offensive grenades can be used by troops in the open, as long as they have strong arms.
- Modern defensive grenades are designed to produce many small, equal-sized fragments with good coverage and not too much range. Some are packed with small balls. Older defensive grenades might have uneven fragmentation with a few larger ones.
Such an older grenade might have a big fragment that punches through the helmet or the chainmail. Small pellets from a modern grenade could slip through the weave of chainmail.
- Depending on the proximity, the blast might have lethal effects as well.
You mentioned a character. Is he in a pen-and-paper roleplaying game? If so, take a look at "generic" games like D20 or GURPS and their rules. Even if you are using a different game, you can play through that one scenario, looking at best-case and worst-case die rolls.