Diana

Diana is Chibiusa's closest confidant. Chibiusa received Diana as a present while young during a Christmas party where she was the Party Star. Diana acts as Chibiusa's attendant and constant companion, with her at all times except in battle and always worried over her safety. She's very sweet even if she is a little too formal sometimes, and she understands Chibiusa's dream to become a beautiful lady, just like her mother. Chibiusa cares about her very strongly and is devastated when Diana is killed near the end of the manga storyline.

Diana worries about Chibiusa so constantly that she is willing to put herself in great danger and go to the past to investigate the disturbances in the time continuum, because Chibiusa herself has been forbidden to go, having already completed her senshi training. Diana begs to go and is allowed, although it is this decision that leads her to be murdered by Sailor Tin Nyanko. Previously Diana had already once taken up the terrible post of time guardian, so that Pluto might go and attempt to reach Chibiusa, who had at that point been turned into Black Lady.

I'm not sure it's possible to be a better guardian and friend than Diana is.