Οικείος is very formal. It is better to use the adjective: γνωστός for everyday speech
Both are included ;)
Why does the μου precede the verb? αυτός δεν είναι οικείος μου was marked wrong. I guess it would literally mean "he is not my familiar."
The short versions of pronouns (accusative με σε το etc. as well as genitive μου σου του etc.) come before the verb.
If you want to use them elsewhere, you'll need the long version: σ' εμένα "to me", for example.
Why is it incorrect to write, αυτός δεν είναι οικείος για εμένα? Even this its the same meaning.. plz fix your app
We would add it, but it's wrong.