Usiel is a prince in the Theurgia Goetia, the second part of the Lemegeton.
The northwest prince
The catalogue calls Usiel its eleventh spirit and the third under Emperor Amenadiel. It describes him as a mighty prince ruling as a king in the northwest, attended by dukes of the day and night.
Source
Theurgia Goetia, the Usiel passage marked 125v in Joseph H. Peterson’s digital transcription.