It seems to me that you are trying to climb a tree from the wrong end,
![Smile [:)]](/emoticons/emotion-1.gif)
as we say in Finnish. To me your most immediate language acquisition activities should entail learning to capitalize
I and all sentences rather than the grammatical finesse your mind is set on.
Of your sentences, the first and the third seem OK to me even though I wonder why No. 3 sounds good, without
an. I can't think of any reason at the moment, perhaps another member can. The second sentence could perhaps be said if "Helena" had been mentioned before, or in the theoretical event that there were several Helenas present and one of them was unsuspecting or in some similar case.
The is more often used when the person is famous:
The great William Shakespeare wrote many immortal plays.CB