I don't think immortality is itself a particularly interesting concept. The compelling aspects of it usually come from what characters choose to do with it, how it impacts their emotional and historical outlooks, and how it affects their interactions with other characters (just to name a few things.) There isn't really any of that here. As someone who has written a bunch of the topic for the sight and generally enjoys examinations of it, I would downvote this for that alone.
The issue at the core of that problem is that we don't get any sense of the character's personality or experiences. It's difficult to care about anything that happens to them without that. We also don't get a good understanding of what put them in this state, denying the possibility for an interesting examination of technology, religion, magic, etc. All that's left is the core concept of being immortal, which as I noted before, isn't super interesting. I would strongly recommend developing out one of more of those aspects if you want to continue with this draft.
Also, I would recommend waiting to receive feedback on your concept thread in the future. It can take more than the half a day you waited, but that feedback will help you avoid spending too much time on a draft when it's being built on shake foundations.
A few specifics for future reference:
SCP-XXXX is free to roam the facility via its choice.
This is a sort of tired cliche, to the point where it was an element in a recent writing contest that focused on overcoming the cliche nature of things like this. At the end of the day, the Foundation exists to contain anomalies, and letting them wander around secure facilities at will runs pretty contrary to that unless there's a very good reason.
SCP-XXXX's height is approximately 169.1 centimeters and weighs 59.8742 kilograms.
Those aren't really approximate numbers. Given the specificity, I'm guessing you just plugged some other numbers into a converter for metric? Approximations are generally two decimal places or less in my experience.
When SCP-XXXX-1 has many other adaptations they are to be named SCP-XXXX-2, SCP-XXXX-3, SCP-XXXX-4, etc.
If you say this, you're setting the reader up to think there's a lot more here than there is. Managing reader expectations is an important part of being an author, so try to avoid implying that there's much more story in an article than there actually will me.
SCP-XXXX has shared an interest in Archangel Raphael often mentioning it aimlessly in an interview.
Is this a pop culture reference or an actual religious reference? It's so out of nowhere that I'm not sure how to approach it. If you're suggesting that the guy is some sort of holy entity, there probably needs to be more hinting of that throughout the document.
Maybe I'll see her again. Maybe Ill never see her again.
This whole bit from his journal is so vague that I'm having difficulty figuring out how to interpret it. While things that spout off nonsense have their place, I've typically found that nonsense is only interesting when it's suitably tangled up in terms of word choice and sentence structure. What's here is just overly vague.
I hope that helps, and feel free to ask any followup questions.
