The term 'porpoise relative' can be answered by two crossword solutions: ORCA (4 letters) and DOLPHIN (7 letters). Orcas, also known as killer whales, are the largest species of the DOLPHIN family and are found in all oceans. Dolphins are highly intelligent aquatic mammals that are found in both saltwater and freshwater habitats. Both orcas and dolphins are closely related to porpoises, which are small cetaceans that inhabit coastal waters.