It is only in the original English dubbing of Sailor Moon in the 1990s and early 2000s that calls Usagi Serena. The reason for the name Serena, instead of Usagi, is likely due to wanting to give her a more English-sounding name. The name Serena was likely chosen because of its similarity to Princess Serenity, which was Usagi’s former name. In the current English version of Crystal, she is known as Usagi.