Aretha is a corporeal spirit located near the soul altar.
To find Aretha, simply travel to Zeah via the southernmost port of Port Sarim, then run north to the Dark Altar via Arceuus House, lastly, run north-east then east of the Dark Altar to get to the Soul altar. This can be done from level 3.
Aretha is the solution to the medium anagram clue: A HEART. The answer to her challenge is 2.