Kei

From Memory Alpha, the free Star Trek reference

Kei (or Braslota III) is the third planet in the Braslota system. In 2365, this planet was chosen as an element in Operation Lovely Angel, a Starfleet battle simulation.

In the exercise, the starships USS Hathaway and USS Enterprise-D were to attempt to defend against an simulated assault on the first planet, Totoro, from forces originating from Kei, while maintaining control of orbital space of the second planet, Yuri. (TNG: "Peak Performance")

[edit] Background

Presumably, this planet lies within Federation borders, but the presence of the Ferengi Marauder Kreechta indicates it could have been open territory. This planet was not mentioned in dialog or the script, but depicted in a piece of background artwork, an okudagram on the observation lounge screen. Most of the simulation information is in-joke references to the Japanese series Dirty Pair, this planet included.