Aegis Fruit

Once your health is capped, every Life Fruit you dig up afterward is dead weight. This is what you turn one into, so the trade only looks like a trade before you hit the ceiling. After that you are converting something useless into permanent defense for nothing.

Flat defense counts for more on the harder difficulties, where each point shaves a bigger slice off every hit, so an Expert or Master run feels the gain most. There is one per character and no way to stack it, so the only optimization is remembering to do it at all.

In Greek myth the aegis is the shield carried by Athena and by Zeus, which is a fitting label for the one permanent booster that gives armor instead of health.