I deliberately left it a bit vague. The human Luna is talking about is either Usagi Tsukino (Sailor Moon) or it's Artemis, but he's a Guardian Cat himself and not a human, hence why I left it a bit vague. If you talk with Duchess and Luna in the Oasis Postgame they tell you how they're both talking about their loved ones and how you saved them. This in reference to Luna canonically having a kid with Artemis, as well as Duchess having three.
So, in the end, I'd say Luna still is the protag's guardian since that's a bond that can't be broken easily. The previous guardian is saved by you and lives (whether that's Usagi or Artemis or both I leave up to your own interpretation) and Luna is also their guardian at the same time.
I hope I answered your question this way! <3