Ekaterina

Ekatrina is Kongou Mitsuko's pet boa constricter. She first appears in the Toaru Kagaku no Railgun episode 13, where a group of girls were invited to model swimsuits, where Mitsuko brought her along 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.