Learn English – Word to describe an attribute one would either love or hate

The most common expression is "unspoken love" (8960 written instances in Google Books). You could also call it "unacknowledged love", but some people may miscontrue that love as only "unacknowledged" by one of the two "would-be" lovers.

I think you want the word 'disciplinarian'. It sounds a bit clinical, but it effectively conveys the meaning of "willing to provide adequate corrective action without undue harshness".

Although it comes from the world of martial arts, 'sensei' is also often used even in non-martial-arts contexts to mean "learned guide who is willing to take disciplinary action for the sake of his charges' well-being".