Bokusenō

Bokuseno (朴仙翁，ぼくせんお) was a wise tree demon who was an old friend of the Great Dog Demon who asked Sesshomaru to stop his half-demon brother Inuyasha. The scabbards of both Tessaiga and Tenseiga were carved from his branches long ago. He can grow arms from his trunk at will.

Sesshomaru came to see him on InuYasha's scent changing to of a full demon twice(whan Naraku's third incarnation broke the Tessaiga, and when InuYasha lets go of the Tessaiga). Bokuseno tells him that InuYasha can never be a full fledged demon. He explains that when Sesshomaru is in battle, that he remains calm, and doesn't lose control.

InuYasha however, when his life is on the line, his demon blood takes over because the demon blood is too strong for InuYasha. As he said, "In further transformations, he would loose part of his human soul. He would not tell the difference between friend or foe. He would only know how to kill until he is destroyed." Sesshomaru understands why InuYasha has the Tessaiga, and he goes to find InuYasha to see if Bokuseno's words are true.

Bokuseno's words repeat in the next episode, and Sesshomaru tells Kagome, if you want him transformed back, give him Tessaiga otherwise he'll keep fighting.