So Rin Hoshizora was created in 2010. Hoshiko is 18 years old, making you (sorry dont want to get your pronouns wrong its rude) born sometime around 1997, about 13 years before the creation of the character. If you identified as Rin Hoshizora is a past life, it must have been pre-1997- before the character was created. So its impossible.
In addition, being fictionkin relies on the ideas of another human being outside of your own. To be consistently fictionkin, you must have been Rin before the character was created and regardless if it was created or not. Meaning you had to be Rin whether or not Rin was made up or not. To be fictionkin, you must have known about the character in the first place, meaning again, fictionkin is impossible.
With fictionkin being impossible, it discredits your notion of being catkin as well. Accordion this this website
https://www.njpetcommunity.com/pets-care-health/how-cats-remember/ : "information can be retained for approximately 16 hours. However, in order for a cat to have such amazing recall of information, the activity or action must be something of interest or benefit."
Other websites also said that it would have to trigger extreme happiness, sadness, etc for the cat to remember something. Or be repeating many times. You specifically said that you dont remember much, which means:
1) you somehow had memory longer than 16 years that lasted through death and rebirth, not even in the same life
2) it must have been something that gave you extreme pleasure or pain or something of the sort, so you should be able to remember more clearly.
Both cases goes undermine your arguments.
Im sorry- I accept the lifestyle and your right to be catkin or fictionkin, and I would gladly be friends with you or someone similar, but I am really having believing in it. And I seriously doubt the credibility.
