A person who uses others for their own gain, only investing in a relationship to get something out of it and not to give, is often referred to as a "user" or "manipulator."
The term "user" implies that the individual is exploiting the relationship for personal benefit, often without genuine regard for the well-being or feelings of the other person involved. Users typically seek to take advantage of others for their own interests, whether it be for financial gain, emotional support, social status, or any other form of personal advantage.
Such behavior is generally considered unethical and harmful to the individuals being used. It can lead to one-sided and unhealthy relationships, where the person being exploited may feel used, unappreciated, or emotionally drained.
It's important for people to be aware of manipulative individuals and set healthy boundaries in their relationships. Building relationships based on mutual respect, trust, and reciprocity is essential for fostering genuine connections and promoting emotional well-being for all parties involved.