Nikosen lived his life as a human monk and began seeking ways to unnaturally extend his life. Eventually he learned to absorb the life energy from trees and his appearance began to change into a more grotesque wooden countenance. His powers have grown weaker and Nikosen must devour Inuyasha and Kagome in order to guarantee his continued immortality.
Sought out by Naraku, Nikosen is given a shard of the Shikon Jewel in return for finding and killing Inuyasha. Nikosen is only too happy to find a demon who will increase his own strength, but unbeknownst to the former monk Naraku's plan is actually a setup. It quickly becomes apparent that Nikosen is not in the same league as Inuyasha and his death is a foregone conclusion. Naraku actually used Nikosen because the powerful spiritual energy he possessed as a human still lies dormant within Nikosen. When Inuyasha uses the Dragon-Scaled Tetsusaiga to slay the hermit, the energy is absorbed by the blade and cures the problematic overflow of demonic energy that the sword had recently been afflicted with.
Through Nikosen's communing with trees he has gained the bizarre abilities of plant life. In addition to his wooden skin, Nikosen can regenerate lost limbs much as a tree would re-grow a broken branch. The monk is also able to quickly sprout root-like tendrils to ensnare his pray and has poisons available to aid in his battles.
The Meaning Behind the Name
Nikosen is written with three kanji which mean "two," "dried up," and "hermit".