A mother's love: Chibiusa|
As stated in the relations section, Ikuko-mommy and Chibiusa are very close. So close that when Usagi asks Chibiusa who she loves more, Neo Queen Serenity or Ikuko, the ever wise Chibiusa says she loves them both equally. ^^
Ikuko takes over raising Chibi-usa for Neo Queen Serenity [well, she doesn't know this, but she does it nonetheless!] when Chibiusa is sent back in time to train with the other senshi, as well as when she initially fell into the past to find help for her mother. Easily brainwashed into believing that Chibiusa was her niece, Ikuko immediately set about to raising Chibiusa in the best way she knew how.
When Chibiusa notices that Usagi doesn't treat her mother very well, she becomes horribly angry as well as confused. She doesn't understand why Ikuko loves Usagi unconditionally, even after she flunks tests, is disrespectful and even says she hates her! Ikuko takes this all in stride and teaches Chibiusa that a mother's love is unconditional and that no matter what happens, she loves Usagi and knows that Usagi loves her. Usagi is just a bit jealous, that's all.
As time goes by it's evident that Chibiusa and Ikuko-mommy need one another. Ikuko's own children are growing up and don't depend on her as much, and Chibiusa desperately needs a responsible adult to look out for her. That and Ikuko-mommy makes some excellent lemon pie. ^^