Responses

Wudo is the demon that came to escort Ryujin and Imae to the institute. You know, the place they were staying when everyone else from the ministry got killed? (and where they first have actual sex)
Wudo appears at the end of chapter 21 (which is also the chapter when Ryujin falls into the secret hiding place of the immortals: aka the accident)
Wait, when did Wudo appear in previous chapters? what accident or fight they had? I'm kinda lost on this part. Someone enlighten me please Σ(っ°Д °;)っ