THE BIT IVE BEEN WAITING FOR WILL COME A LOT SOONER THAN EXPECTED!
In the Saiyuki manga, Hakkai had taken off his limiters three times - once in the Burial Arc to save Gojyo from a group of youkais who held him hostage, another time in the Kamisama Arc before Goku while they were trapped in Kinkaku's gourd, and a third time in the "Even a Worm" Arc in the Reload manga, where he had to stop a berserk Goku as Seiten Taisei. As mentioned previously, he also took his limiters off in the Homura Arc in the anime.
Until recently in the "Even a Worm" Arc, Hakkai's transformations reveal only a back view and a blurry profile of his youkai self. Very limited information is given regarding his conditions as a youkai. The "Even a Worm" Arc clarifies a lot of things. The story was that Goku was attacked on the streets by an unknown foe and was badly injured, to the point where Hakkai declared that the only option was to take off his diadem and hope that his Seiten Taisei lifeforce can save him. In the meanwhile Sanzo had gone off by himself to find the one who attacked Goku (and kill them). As Goku recovered and turned berserk, even Gato's bullets could not stop him - he caught them all. Deciding that there was one available option to take, Hakkai took off his limiters, asking Gojyo to stop him if he goes berserk.
Now we are provided with a bit of background information in the form of a flashback of a conversation between Sanzo and Hakkai. Sanzo warned him that Hakkai should not take his limiters off - that he, unlike Goku, would be affected by the minus wave, and possibly lose his senses for good.
This is also the first time where the audience can see Hakkai's face as a youkai: he has the typical pointed ears, and his hair gets longer - Gonou's hairstyle, basically. His vine markings wound around the whole of his body, even his face (in the manga, the vines are green; in the reload anime they are red). And as the solution for the much-debated topic of Hakkai's eye in the manga, here Hakkai's right eye is clearly shown to be prosthetic; his left eye becomes golden with a slit-pupil, while his right remains green with a round pupil.
As Hakkai took on Seiten Taisei Goku, he is shown to be much faster than his human self, and much more durable. However, as he was subjected to the minus wave, he was shown to be sadistic, as he repeatedly punch Goku in the face whilst holding him down. As Gojyo held him back, Hakkai's fist almost landed onto Gojyo's face before he restrained himself and regained some human senses. He himself was shocked by his own brutality under the effects of the minus wave.
Another exciting feature of Hakkai in youkai form is his markings. They are much more extravagant and dynamic than those of normal youkais, and for a good reason - one of youkai Hakkai's ability is controlling his vine markings and have them travel across the surface of any solids in order to bind. He used it to grab hold of Goku as Goku landed onto the ground, grabbed Goku's arm from having them travel out from his ankles, and to conduct electricity through Gato's gun, Gato, Hakkai himself, the ground and Goku, all bound by his vine markings. This skill in its own nature seems to startle and even shock the Seiten Taisei Goku.
However powerful Hakkai is shown in this battle, it is also made clear that he is no match to Seiten Taisei Goku. Goku is a lot more agile, faster, more brutal, stronger and recovers at an unbelieveable rate, and he can also utilise the energies of the earth. Hakkai only managed to restore Goku's diadem through manipulating the natural power of lightning, and he almost killed himself in the process.
I THINK IM IN LOVE