The 'newcomer' character should not — repeat, should not — be an already-existing one. The whole point of this "intro to Metamor Keep" story is that the reader doesn't need to know anything about the convoluted historical/political/interpersonal relationships that have already been established for MK, and that's exactly why it's a mistake to cast Tessa in the role of 'newcomer to MK'. What you want is someone who's a complete, utter and total blank slate, as far as the setting is concerned, not someone who's already got X number of stories under their belt! Again (because the fact that you guys are actually considering Tessa at all means it should be repeated): The whole point of this intro story is that you don't need to have read any other MK stories before you can understand what's going on. Bad, bad idea!
As it happens, I have a character concept that might be suitable for the 'newcomer' role in this story. I rather doubt I'm ever going to write any MK myself, so I'll just offer up this character concept as 'community property', for any MK writer to use or ignore as they see fit. Yes, this means I officially renounce any claim of ownership — if it matters, I guess Chris O'Kane is the one to talk to about using this character or not.
Anyway, here's the character concept…
The guy (who may or may not actually be male, but I'm gonna call it "he" rather than use the indeterminate 'they' for one specific person) is a human scholar whose specialty is [insert appropriate flavor of magick here]. He's devoted most/all of his adult life to his studies, and so he's pretty damned knowledgeable. Of course, that means he's also getting on in years… He's heard of the Curse, and he finds it very interesting; not only do animalized Keepers seem to live a lot longer than their 'natural' counterparts, but there's also the AR-flavored variety of Curse. In other words, the MK Curse can do both rejuvenation and life-extension — both of which topics our boy is seriously interested in, thanks to his advanced age. Such a pity that those benefits come in a package deal with some rather undesirable side-effects…
The guy is a scholar of magick; he devises what he thinks is a way to control the Curse, and be subjected to only those specific effects that are wanted while not suffering any undesired effects. It all looks good on paper… but for some reason, nobody's willing to volunteer as a guinea pig to test the guy's theory. And in the fullness of Time, our boy decides to be his own test subject. His life is nearly over anyway; worst case, he really doesn't have much to lose. And there's always the possibility that it will work!
So our boy travels to Metamor. He is fully aware of the Curse, and he fully intends to get hit with it. As well, he's got some sort of ritual or amulet or something which, he hopes and believes, will control his particular Curse so that he ends up being younger, and that's it. While waiting for the Curse, he busies himself comparing notes with Metamor's own scholars; he might even interview random Keepers, focusing on their particular Curses; basically, a great opportunity for infodumps that seem natural, as opposed to being heinously obtrusive.
When the Curse hits our boy… well, he got what he wanted, sort of. He ends up as a shrew. For those of you who don't know: Shrews are an extremely small group of mammals, the largest of which are 15 cm long and weigh 100 grams, with the smallest being 3.5 cm long and 2 grams in body mass. Their high metabolic rate that forces them to eat 80-90 percent of their body mass in food every day. They are generally hyperactive and cannot hibernate. Oh, and some shrews are venomous (a rarity among mammals)…
But I digress.
Anyway: Shrew. Our boy is about 1 foot tall, weighs around 10 pounds, and must consume at least 5 pounds of meat every day just to stay alive — more, if he's expended any significant amount of muscular effort. Vegetarian diet ain't gonna work for him, 'cause he needs the concentrated protein/calories/etc of meat. And thanks to his fuel-injected metabolism, he lives about 2* or 3* times faster than everybody else; as you can imagine, this is a bit of a barrier to communication, albeit not an insuperable one. For one thing, there's always the written word.
In short, our boy wanted extra time… and he got it, good and hard.
Like I said, this character concept is open to anybody who feels like writing about it. Go ahead — I won't mind. Promise!