Sein

An infiltration-type cyborg created at some point after Quattro. As the name suggests, her Deep Diver 'IS' allows her to swim through inorganic objects (like the ground or rocks) as if they were water. The Periscope Eye Inherent Equipment is a cybernetic apparatus in her right index finger that acts as both an extra eye and as a tool to disable electronic and magical locks. Sein is one of the most cheerful, yet suspicious-looking among the Numbers. Although she showed many failures in the past, her unique 'IS' makes Sein a valuable asset in both Scaglietti's and Uno's eyes due to its completeness and independence.

As she stand in the dead center of the order that the Numbers were created, she normally acts as a 'bridge' between her older and younger siblings. Sein and Wendi get along pretty well because of their similar temperaments and the fact that they were tutor and pupil for a while. In episode 24, Schach Nouera knocked her out. (Source: Wikipedia)