A criminal's heart will always be sad. No one chooses a life of crime if they're happy. And even if they seem like they're doing fine on the outside, deep down they're really sad.
The sad feelings fill the criminal's body with heat (ひ). This heat spreads and radiates out. If you walk next to them on the street, you'll feel the sad heat floating by you.
Currently there are no similar looking kanji which exist in the WaniKani learning system