The Wisp is part of the builder techtree and evolves into more powerful versions of itself (Wisp 1-4, with ultimate upgrade making it a Wisp 5 with more building options), a giant tree hero with ridiculous HP, or an Ancient - who had a lot of HP and Defense as well as some useful abilities.
Regarding the Wisp itself It can build farms, hunters dens, ancient of protections, and ancient of wars, and moon wells (they upgrade into better moon wells or into the hunters den). when fully upgraded into Wisp 5 the wisp gets the chimera roost and an altar of nature, which can train a very useful hero.
Wisps Offensive and Defensive Capabilities
Wisps sport a ranged attack that does relatively low damage, they boast a few useful spells that can help them escape from bad situations; these spells being mirror image (the images do do minor damage) and blink.
In terms of units the wisp has (from the ancient of war) Archers, Huntresses, and Ballistas. they also have Keepers of the Grove available once they become ultimate, as well as Chimeras from the Chimera roost.