"Ghost Viper" (Yūrei Mamushi in Japanese) is the alias of Shigeru Sano (or, rather Sano Shigeru for proper surname-first order). He is a Japanese crime-lord who decides to “supernaturalize” the Yakuza, and sees Darkstrife and Painstake as a potential power source he can harvest, and obsesses over obtaining them to cement his power.
Sano has “ghost scars” in addition to his pre-existing Yakuza tattoos. He has lost most of his forearm and his left hand. However, he’s using the power he’s accumulated to make himself a “ghost hand” by capturing and binding a ghost to him. He is technically haunted but in control and uses the ghost's hand as his own.
Ghost Viper is no longer actually Yakuza. He was a lieutenant, but was disgraced because he was seeking these alternate means of power (human sacrifices, etc.) and was ordered executed. He managed to turn the tables on the Yakuza that were supposed to kill him and is now trying to set up his own syndicate - the Yokai - to fulfill his goals.
His first appearance is in August ’93 as Ghost Viper (MC vol. 2 #240). The first appearance of the Yokai is in the previous issue.
He’s a ’90s character, but the idea of the heroes having to fight “demon ninjas” goes back into the late ’80s. However, the question that prompted of “wait, where did the ninjas get demon powers?” is what results in the introduction of a separate organization (the Yokai) and then Ghost Viper as the individual behind that organization. [02:20] Ghost Viper and the Yokai are also a response to Zhu Long, but are more internally consistent in terms of being Japanese rather than Zhu Long's pan-Asian mishmash.
Ghost Viper therefore covers some of the same ground as Zhu Long, while maintaining distinct. He is a source of ninjas, oni, and vaguely-demonic thugs. Sano is in this for his own personal gain and is pettier than Zhu Long, who allows him to exist in this same “area” because he’s a useful idiot : it provides cover for Zhu Long more secretive activities if Ghost Viper is out there running a similar shtick.
He as a decent variety in “scale” of the jobs he’s pulling, but Sano is still not really “running” a large chunk of Tokyo yet. That’s his end goal, and he has successes, but they’re less “I now control Tokyo” and more “After burgling that private collection, I now have the [MacGuffin of the week] that gives me new powers!”.
He can give out temporary “buffs” to people, and that’s a source of one-off foes for Painstake and Darkstrife. He also can give his underlings more permanent abilities making them more interesting to combat.