Vital Crystal

Health does not come back at a flat rate in this game. It ramps, so the longer you go untouched the faster the ticks arrive. This crystal speeds up that ramp rather than adding a lump of healing on top, which means what you feel is reaching full regeneration sooner after a hit, not healing faster while standing still. In a fight where something clips you every few seconds, the early part of that curve is the only part you ever reach, and that is exactly where it helps.

You get one per character and it costs you a Life Crystal to make. Spend it after your health is already capped, because hearts are common enough by then that giving one up costs nothing. Doing it early means walking around with a smaller health bar for a benefit you will barely notice.