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 Jail Scaglietti's and Uno's eyes due to its completeness and independence. As she stands 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. She likes to pull pranks on others by her using her Deep Diver ability to grope people.