One of Three Great Dojutsu, Byakugan has played an essential role in the shinobi world of Naruto. It originated from the Otsutsuki clan and after their arrival on earth, the Hyuga clan has been commonly associated with the Byakugan.
In the new era of Naruto, Hanabi Hyuga has become the head of the Hyuga clan succeeding her father Hiyashi. Toneri Otsutsuki kidnaps Hanabi to steal her Byakugan and described it as the purest.
How Hanabi Hyuga's Byakugan is the purest in Naruto
Hanabi’s eyes are considered to be the purest form of Byakugan and the only person who described its value was Toneri. He is the descendant of Hamura, also the only survivor of the Otsutsuki’s branched out clan on the moon. His Byakugan was sealed within the Energy Vessel by Hamura.
In The Last: Naruto the Movie, Toneri specifically targeted Hanabi's Byakugan in order to awaken his Tenseigan, and described it as “very pure.” The reason behind it was her being the closest descendant of Hamura. Moreover, Hanabi claims that her Dojutsu’s perception and visual prowess are far greater than any other fully grown Hyuga.
Toneri, with this awakening, was capable of destroying the earth by colliding the moon on its surface. As the Dojutsu allows the user to control both attractive and repulsive forces with additional abilities like increased strength, enhanced sensory perception, and durability.
Although, Tenseigan can be awakened if an Otsustsuki is implanted with the Byakugan of a Hyuga. Hamura is the progenitor of the Hyuga clan and he was the first to manifest Tenseigan. A Hyuga can’t acquire Tenseigan, because being a descendant of Hamura is not enough, one must also possess Otsutsuki’s DNA or blood.
Unlike Mangekyo Sharingan and Rinnegan, a Tenseigan can’t be implanted as it will lose its potency after the removal. As seen in the movie, after Hinata defeats Toneri, she retrieves her eyes back and implants them back into Hanabi, but it returned back to its Byakugan state.
Hanabi sure is the direct descendant of Hamura and has the purest form of Byakugan, but she doesn’t possess Otsutsuki’s chakra, which is a vital element in awakening Tenseigan.