Mage: One who guides others with knowledge of their aspect. They’re similar to seers, and also have a talent for hearing their exiles exceptionally well. While Seers gain knowledge about the universe using their aspect, Mages are those whose aspect gifts them knowledge of itself/is gifted knowledge of aspect by the universe. This means Mages will always have an in-depth understanding of how their aspect actually works and can use that knowledge to guide or give their team warnings. They lose/sacrifice a part of themselves in exchange for lacking a core part of their aspect within themselves, which gifts them protection from their aspect.

Hope: This one is self explanatory. But also tied to belief in general. If you believe in something hard enough it might happen. They would be magicians in an rpg, as they have an immense amount of magical power. Holiness and angels are also representative of this aspect.

A Mage of Hope would probably be the least overpowered Hope class, but that doesn't mean they couldn't be a useful or even a vital team member. "Least powerful Hope class" is still going to be a powerful class. Their understanding of Hope would either come from an intuitive understanding (their own Hope), or from contact with powerful forces within their session who educate them on it.

The most important thing that they would understand is that belief is key in all things. They would guide their team by drilling it into them that the more faith they have in anything they're doing, the more likely they are to success. By guiding others towards hope, a session with this mage would have every player be able to harness the power of Hope to a small degree.

They would also have the ability to tell when a situation isn't as hopeless as it may seem. If their teammates have all given up, a Mage of Hope would lecture them and guide them away from their pessimism.

When they go through the ordeal of their sacrifice, their protection would likely come from a natural avoidance of ever having hope in something truly impossible. If something is truly hopeless, they would warn their team. As for lacking a core part of their aspect, I believe they would be the only one on their team that can't wield the power of Hope to any extent. While they understand how powerful belief is, their own belief lacks any power.