Why is Futaba's nickname "Teko"?
In Japanese, Hikari says that Futaba's eyebrows look like tenten, the two strokes added to a kana to change the voicing value, also known as dakuten. E.g. か ka becomes が ga.

You can see the resemblance in this picture from the manga, at least if you imagine Futaba's eyebrows both pointing in the same direction:





"Dotty" came from a translation of tenten as "dot", presumably because "Periody" or "Semicolony" didn't sound good.