Ekaterina

Ekaterina is Kongou Mitsuko's pet boa constrictor. She is taken along by Mitsuko when she was invited to model swimsuits and shows her to the rest of the group while showing herself off in her swimsuit. Most of the girls were freaked out, but Uiharu took an immediate liking to the snake. Later, Mitsuko mentions her while she was under MAR's custody and comments that she'll get worried if Mitsuko doesn't come back soon.

She is once again mentioned by Uiharu while she, Ruiko, and Kuroko were investigating the "Someone's Watching" urban myth.

She is mentioned by Mitsuko when she buys food for her at a pet shop after arriving in Academy City. She also appears with Mitsuko in the opening panel of Chapter 40 of the Railgun manga.