So I had a little chat with the account borrower.
For the story, I've given the man an other account, on may 2014 (with login / password), he asked me for it and I wasn't playing, so I gave him.
Then, later, I've made myself a new one to play again, but used the same password I always use on H&H

.
Later, the guy apparently looked for other accounts of mine on the forum found this one, and tried the password I gave to him earlier, and he was in...
So I've made a mistake and now, obviously, I've changed my password.
Though, taking advantage of something I gave willingly to sneakily access my other accounts is not nice.
But then my main didn't die in the process, and have been well kept. So all in all it's not that wicked neither.
Now this is still my account, and I'm having it back.
And that's still my characters so I'll also keep what's on them (as he kept what was on them when he "took" them for himself).
So yes, case closed I guess.