As it stands, she had a function in the plot, exactly as Mrs Snowball, or Mrs Finch or Mrs Alcazar or any of the other passing ladies
Good points and yes I agree, actually now that I understand. Herge didn't intend it because he didn't write the story in it's entirety. However, I will say this.....it's not a common accurance in Tintin, that a girl asks him to dinner to meet her parents. A young girl.
Now this is just before the story ends.
Take the new Star Wars film for example, George Lucas is sliced from the production of the new stories. He has no say at all, but the story as it is now, is considered canon, even though the creator of the saga isn't attached to the product as a whole. Not entirely anyway. So.....maybe Herge didn't intend for Martine to be a love interest, but there is something odd about how they ended the last story. It is a nice ending that fans can put together in whatever way they want. Personally for myself, I think the intention they were driving towards with Martine was....ok this girl clearly has a thing for Tintin, whether he is interested or not, we don't know
....and we never will know for sure, because it's left open ended, but the impression that was left was she asks him to dinner, meet her parents, young woman....just the impression I got anyway and it is a nice ending.