Sakamoto

A black cat who wears a scarf created by the Professor, which allows him to speak. Being theoretically older than both Nano and the Professor when converting cat years to human years, he speaks in a condescending tone towards them and wants to be called "Sakamoto-san", though he often succumbs to his catlike habits.

[Spoiler] In the last episode, it is revealed that Sakamoto used to be Nakamura's cat and was called "Taisho". Sakamoto does not want to return to Nakamura because she did not take good care of him, even though she had good intentions. [/Spoiler]