"Who is he?"
Translation:Ποιος είναι αυτός;
In this sentence, yes. Ποιος είναι; translates as "who is it?" (in the case someone knocks your door for example and you do not know the gender). So who is he? is ποιος είναι αυτός; to specify the gender. ποιος είναι αυτός is also "who is that/this" for males.
Even in context? e.g. "There's a strange man in an orange wig over there. Who is he?" ?
Why is there no stress mark in ποιος? Is it considered one syllable?
Yes, it's just one syllable, roughly pronounced like "pyoss" (IPA [pços]).
What do you mean with "words relevant to gender"?
All Greek nouns have a gender, so gender is incredibly relevant throughout the language.
Oh....that is what I meant. Because some languages, i.e. English, don't have words that have gender, but some languages like Spanish do.
Yes, Greek has three genders: masculine (e.g. ο άντρας - the man, ο αέρας - the air), feminine (e.g. η γυναίκα - the woman, η πόλη - the city), and neuter (e.g. το παιδί - the child, το πάτωμα - the floor).