I don't mean someone who lacks emotion because they "don't care", but because either they can't feel emotion or the emotional response is delayed because of a genetic disposition.
Maybe there is an appropriate medical term that could be used.
The word stoic is recommended from a similar question, for example – but that question and its answers relate to an individuals ability to endure/tolerate a situation or simply ignore their emotions.
Best Answer
Blunted affect may be the noun, but if you're looking for an adjective to describe someone like that the term is affectless.