symbiotic

A symbiotic relationship benefits both parties. The two of you have a symbiotic relationship: she fixes your car and you style her hair.

In biology, symbiotic refers to any diverse organisms that live together, but in this case, the relationship is not necessarily beneficial to both. Parasites, for example, have a symbiotic relationship with their hosts, but only the parasite benefits. If you contract a parasite, you might consider charging rent or throwing the bum out!