Seishirou Shishirui

Shinobu's first minion and the original owner of Kokoro-watari and Yume-watari. While extensively referenced to throughout Onimonogatari and directly encountered by Koyomi, Suruga, and Shinobu during Owarimonogatari, his original name isn't known until it is revealed by Ougi to Koyomi at the end of Shinobu Mail.