Diana

Diana was a small gray kitten. She was the future daughter of Luna and Artemis, and served as Chibiusa's companion.

Like Usagi and Luna, or Minako and Artemis, Chibiusa and Diana shared a fairly close bond, and she could often be found riding on Chibiusa's head.

Manga
Diana appeared significantly earlier in the manga than she did in the anime. She appeared during the Black Moon arc, which corresponds with the second storyline of the R season. Like her mother and father, she had the ability to take on human form in times of crisis, though unlike them she retained her tail. Like Chibiusa, her hair was bound in odango on the sides of her head.

She was friendly with Sailor Pluto, due to the fact that Pluto and Chibiusa were friends, and guarded the Space-Time Door for Pluto when she convinced Pluto to join the battle against the Black Moon Clan.

Anime
Diana did not appear until the SuperS season in the anime. The anime did not show any evidence of her bond with Sailor Pluto, nor did she ever take on human form or guard the Space-Time Door.