Changelings in mythology were creatures placed in baby's cribs to fool parents when swapped with a human child. Another possible interpretation is that of a Doppelganger, or a Shapeshifter. This fictional, and primarily female, creature can change form and take the place of any one she sees.
The Changeling can copy any ability in the defending village, making her a valuable tool if used correctly.
Try to target villages with units that will make her more powerful when attacking. The ability can be chosen *after* a defending unit is decided, unless the chosen unit's ability states otherwise, such as with the Duelist or Kaiju.
If you fight Medusa when attacking with a hired Changeling, you can copy her ability and destroy another card in her village.