Derane 344 was a Yeerk who was left for dead when the Animorphs destroyed the Kandrona. Eslin 359 wanted to kill Visser Three to avenge her death because she was the only creature in the galaxy that he cared about. Eslin and Derane came from the same pool and went through training together. Derane was considered 'expendable' during this time of Kandrona scarcity because she was given a minor post and deemed a less important Yeerk by Visser Three. Eslin discusses this with Ax right after he discovers Ax calling his planet at the radio observatory.