Not quite sure what is meant by mysterious here.
If it is something not clear, evasive, illusive, then it would be Neptune.
Your 7th house symbolises the type of partner you attract and need, and mercury is near the cusp.
Mercury in itself is "mercurial", changeable, and it is sextile Neptune.
A changeable partner who is hard to put your finger on, to pin down, because they are illusive and "mysterious".
Not necessarily a negative quality.