What are 'Missing hearts' ?

[Andrius227]

Leech companions give missing hearts. I thought the nurse at the inn could heal them but she cannot. So how can i use these missing hearts?

https://steamcommunity.com/sharedfiles/filedetails/?id=3305418246

[Eonfighter]

Don't quote me on this, but I think that those simply count towards traits or items that care about "missing hearts". Effectively, the items that say something like "1.1x damage per missing heart" are impacted by the missing hearts added by the leech. Haven't tested this theory yet, but that's how I interpret the term.

[Highborne]

A few items give "unusable" hearts (blood vial companion is the main one), which count towards missing hearts. Also empty red heart containers are also missing. This is useful for a trait and an item, but is very underutilised right now.

[NadirehS]

I wonder if these are usable to open blood chests