H-Nothing is Hardened-Nothing. As you might or might not remember, each attribute can have an attitude which can be either “good” (Empowered, Hardened), “neutral” (Inception, Gathered), or “bad” (Lost, Scrambled).
Inception (I) = starting attitude; can have Empowered or Lost tendencies but can’t control either very well
Empowered (E) = got control of a Directed attribute, confirmed for hero, everything is peachy
Gathered (G) = has given up figuring out how to be Empowered in exchange for not being Lost; typically a person is Gathered in both a Directed and Undirected attribute so each keeps the other from progressing
Lost (L) = Undirected attribute controls you (ex.: L-Portent might be frequently overcome with frightening intrusive thoughts)
Scrambled (S) = Undirected attribute consumes you and causes you to torment others (ex.: Aiyalam is S-Incongruity so he’s a little bit crazy and his thing is basically screwing everything up)
Hardened (H) = managed to come to accept an Undirected attribute and actually turn it around to be useful (ex.: H-Compulsion is good at feigning a fake set of thoughts/opinions that people expect, while actually having another entirely)
In-universe, Stolen Heart is the main guy who’s H-Nothing and in his case, he has no emotions, no identity, no capacity for romance (at least seemingly, it’s hard to tell without emotions) and basically literally nothing to live for but he’s realised that that instead means that without emotions he can’t be biased and can basically do perfect science emotion-wise, and nobody can bring him down because he has nothing to take away.