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
In the manga, Diana appeared significantly earlier 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 Diana convinced her to join the battle against the Black Moon Clan.

Anime
In the anime, Diana did not appear till the SuperS season. 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.

Video Games
Diana appeared very briefly in the opening of the video game Sailor Moon: Another Story, along with with King Endymion and Neo-Queen Serenity.

Mythology
Diana is named for the Roman goddess of the hunt, who eventually replaced Luna as the moon goddess in Roman mythology. Diana was the equivalent of the Greek goddess Artemis.

Trivia

 * The name "Diana" was also used for a fairy in prototype drawings by Naoko Takeuchi. This "fairy" concept was eventually scrapped.
 * Diana was always very polite and formal, and referred to Usagi and Mamoru as "Serenity-sama" and "Endymion-sama" ("sama" (さま; "様" in kanji) is a non-gender-specific suffix to people's names for showing respect).