"Toxic” isn’t exactly the word I’d use either way, but to your point, it depends on what you mean when you say you “kin” them.
If you mean you identify as them on an integral level, then you’re more or less fine. A lot of otherkin don’t like hearing “kin” used as a verb - myself included; I feel rather strongly that it’s a big contributor to the misuse-of-language problem I’m about to explain - but you are actually fictionkin, so while I would encourage you to stop using it as a verb both for misinformation’s sake and because it will make a lot of other actual fictionkin avoid you because they’ll think you’re misusing it (again, more on that in a minute), if you really insist on using it as a verb, that’s your business at that point, I’m not gonna harass you about it.
If, however, you mean... pretty much anything else, most commonly one of the following:
You relate to them very strongly
You like projecting onto them (and may use it as a coping mechanism)
You stan them/really really like them
You like roleplaying as them/using them as faceclaims (and may use it as a coping mechanism)
It’s a form of role play (with a space) for you, as explained here
and you don’t actually consider yourself to be them on some level, then you are one of the people referred to in the OP as “KFF”, which is a shorthand for “kin-for-fun people”, and are, probably without realizing, misusing the word “kin” to mean something it’s not.
That’s probably not really your fault, and no shame if you didn’t know that up to this point - the “kinning is relating to something” misinformation has spread really far at this point, and for a lot of people it’s unfortunately their first exposure to the term “kin”. What happened, as far as I can tell, is essentially that people stumbled across light-hearted, jokey posts from otherkin and fictionkin and misinterpreted those posts as being representative of the whole experience, and thus misunderstood what otherkinity and fictionkinity are to begin with.
The problem is that they then started spreading the idea that that’s what “kin” means - that it’s a fun fandom thing that just means really relating to something, instead of an actual identity that is integral to how one understands oneself - and now we have a problem where not only is it incredibly difficult for fictionkin in particular to find actual other fictionkin, a lot of people who “kin for fun” outright bully actual otherkin and claim that we’re stealing and misusing the terms we originated over 30 years ago, that if you actually identify as your “kin” you must be “delusional”, etc. - which, we’ve had a problem with antikin for forever, but it... it hits different when it comes from someone who’s actively taking your terms and trying to shove you out of your own community, especially if you thought they were one of you and that you were in a space that was safe from that, y’feel?
(If you’re still lost on how this is harming the otherkin community, more on that here, here, and in my “kin for fun” tag, which has a whole bunch of posts both talking about the subject in general and anecdotes from various people recounting their experiences with how the phenomenon of KFF has harmed them personally.)
So... yeah. If you actually do identify as these characters - or if you think you might and aren’t sure! no shame in that! - then you’re pretty much in the clear, though personally I would encourage you to quit using “kin” as a verb. If you’re using it to mean pretty much anything else, I would ask - beg, even - you to please stop doing that and use another word. If none of the words “stan,” “relate to,” “mood,” and “roleplay” don’t fit perfectly or don’t vibe right for you, there are several alternatives that have been coined specifically to replace “kinning” as a word, which can be found here. (Or coin your own, if none of those vibe right for you! We are all about creating language to describe your experiences where no words exist here - that’s how the word “otherkin” came to be in the first place, after all.)