I think the key is what FantasySciFi wrote:
The key for most identification is setting up visual anchors throughout the script. Making it very innocuous, have him waiting for her or reading and set down a National Geographic which has a panther on the cover. Later he and she are walking past a store or are sitting at home and they flip on the TV and there is a nature documentary with panthers. At some point later, a black cat jumps onto his lap that he strokes.
Just like in advertising, there are now three links to panthers though it was all suggested subliminally.
So in that case it's all about foreshadowing. For that you can't hardly beat wheatgrinder's suggestion:
purely expository approach might work here..
They panther character in human form is talking about.. "when I die, I want to come back a a panther"
Boom done.
Having no idea what sort of things you have room for in your story, can you drop in or pepper your story with these sort of foreshadowing elements in order to lead your character and audience to make the connection you want?
Well, how many times have I heard friends and family say something like, "Yep, I think it was Mom (deceased) watching over me that time," when something beneficial or fortuitous happens. They like to think that it wasn't simply chance, but rather some passed loved one guarding them or actively helping them out. So I think that the good news is that people are inclined to make those sorts of connections. If your story allows for that kind of magical thinking, which it sounds like it does, I hope it wouldn't be too difficult to get what you want.
So, reading this thread I was trying to think of different foreshadowing elements that might work to lead your audience to make that connection.
Earlier in your story, is it possible to have one of the shared experiences your couple has be something like going to a spiritualist, perhaps on a lark on a fun day out together, in which the spiritualist tells each of them (or at least him) what their spirit animal guides are (ala something like Clan of the Cave Bear, for example...not that they need take a grueling hallucination inducing hike to the top of a magic mountain!)? Maybe they even have a dialogue about it afterwards. Maybe, he likes that his is a panther, and, like wheatgrinder wrote, he even says he'd like to be reincarnated as one.
Maybe we see that he has a panther on his keychain. Maybe he has photos or paintings of panthers around his flat. Maybe conversations happen about them letting us know he loves panthers or wants to come back as one, etc. Maybe at some point he describes -or you show- how as a child an interest in panthers was impressed upon him.
Anyway, that sort of foreshadowing might be the way to go.