We've already talked about a few options, but the best ones so far seem to be Belen and Stian.
Stian -- Stian is from South Glacies and currently on a personal quest to become a stronger Health user before heading back to help her people. She's currently in Sera and could probably find a way up to North Glacies. She'd probably feel obligated to see if she could help a bit, especially since she's already on the other side of the world from S. Glacies. Her plan as of this moment is to head back south when she's achieved Master, which will be a little bit.
Her current arc, in my mind, is a transition from being all about helping the injured to actively trying to stop people from getting injured in the first place. Basically a transition from passive to active, in the conflict.
Belen -- This lady is from a small Undan fishing village and wants to travel, explore and not be tied down. She's a Water Master but her abilities are not offensive at all, and mostly have to do with sensing where things are in water and taking salt out of water.
She's much more open-ended. She doesn't really see herself as part of the Glacies/Unda conflict at all, even though she is by association. It'd be cool to get her and Martyn in the same place to explore that. Plus, how he handles an Undan that can really do him no harm.
We also talked about about Elias, though that needs discussing with Milk, and Errol is up for stuff if Martyn is ever in Ignis. They can be Rebellion Bros (though their movements have very different stakes and technically Martyn is a leader and Errol is a bruiser).