Neptune in 1st House synastry is a promising, but very difficult aspect to establish a sincere relationship. Neptune distorts reality between us, but it does so differently for you and your partner, and a lot depends on the natal aspects of Neptune in his or her chart, and thus the natal aspects of your First House.
For example, if your First House is damaged, it can mean your reactions will almost always be negative: you will constantly take everything in the worst possible way, or even worse. And your attempts to sincerely explain your position or condition will either lead your partner into ecstasy (with a harmonious Neptune) or throw him or her into a total bewilderment. Regardless, in order to learn to properly perceive your personal manifestations, your partner will need a long time to learn; and the question whether or not you are sincere will often be a true stumbling block for them.
Neptune in 1st House synastry, especially if Neptune and your First House aren’t very strong, tempts you to swim according to the whims of fate, not by building relationships, but by rejoicing in their bright moments (of which there may be many at first) and overlooking the contradictions, especially since it can be very difficult to resolve them. This path, however, leads partners to bind themselves in mutual lies, which are increasingly obvious, while joyful moments are gradually disappearing and the meditations become more and more crude.
Generally, on a low level of spiritual evolution, Neptune in 1st House synastry is a dark aspect: both you and your partner have the constant temptation to deceive the other party, except he or she misleads you superficially, and you mislead them deeply.
from A. Podvodny
Neptune in 1st House Synastry Explanation
Under Neptune in 1st House synastry, a strong psycho-emotional relationship is established between individuals, forming their further relationships.
The intuitive possibilities of foresight arising at the subconscious level of the “Neptunian” personality have a constructive or destructive effect on the possibilities of self-expression of the person of the “First House”. If Neptune is positive, the “Neptunian” personality significantly increases the level of spiritual and emotional perception of the personality of the “First House”, allowing it not only to improve, but also to constructively express itself, and this, in turn, stimulates the “Neptunian” personality to reveal more positively and actively their intuitive, aesthetic and creative abilities.
With a damaged Neptune, the “Neptunian” personality negatively affects the psycho-emotional state of the personality of the “First House”, condoning its bad habits and inclinations while it begins to drown in empty and useless fantasies. If the partners are close, or locked in a family-household relationship, all of the above becomes even more evident.