Seno (背の Senou) is the pet of Mashiro together with Uno who combines with her to get Mashiro in her Taller Form. Seno and Uno are called sheeps by their owner, though they look like Chicks. Seno is one of the main protagonist of the series, Kamikami Kaeshi.
From the name Uno, means One or First and Seno, which means Of The Back. This overall means that Uno is the first and Seno is a last.