Previous Versions can be found here.
Diamond Boy 1 is a very useful card added in Beta 6.6, giving what is basically Armor to non-Armor monsters on your board. It won't affect itself, so a 2nd DB1 is needed to give each other "Armor" (but the effect doesn't stack on the other monsters).
With just 1 less DMG being taken, effects like those of Undyne are entirely denied. And monsters like Mad Dummy need to be struck with a higher initial damage number, which becomes rarer very quickly, keeping it alive more often.
Since Beta 54.0, it has Taunt, making it unable for people to hide it behind other Taunt monsters, which often forced spells and effects in order to deal with DB1 before the Taunt.