Mitsurugi no Mikoto, Saji
Full art:

Appearance:

Left: “Shell-shaped shoulder armor”
Right: “Reptile legs”

Snake:

“The snake on its tail in dragon-mode moves to its left arm while in human-mode (can move across all of its body)
During a slashing attack, it can move to the sword and jump out”

Futsu no Mitama no Mitsurugi
Full art:

“The tip of the tail has been possessed by the flame snake”

Source.