I have an idea for an ally of humanity (we obviously wont play as one), I got it from a book I read a while ago, let me write it up and see if it fits.
edit: here it is
Suggestion for an ally of humanity, taken wholesale from a fantasy novel
Dragonlords aka Weredragons
Dragonlords are born with a split soul, half dragon and half human, until an adult (earliest is about 20) they are indistinguishable from a normal human. When “first change” comes upon them, they have an irrational urge to get out into the open, where they change into their dragon body for the first time.
Dragonlords have made themselves guardians of humanity, in the book this basically means that they are arbitrators and judges, as most of the issues facing humanity come from within it. For this game however the threat is an outside force, thus they would be free to act as warriors.
Dragonlords are “given to eachother” before they are even born, they instinctively do not form romantic attachments until they find their Soul twin
Abilities:
Longer life span
Enhance strength, speed, visual acuity
can summon “coldfire” which provides illumination but not heat, normally pure white but can be coloured at the dragonlord's discression
Immune to all non-magical fire and heat
can communicate mind to mind, and allow others to respond
Able to transform into a (small – still a fair bit bigger than a human though)dragon, takes approximately 3 seconds, and can be done at will unless exhausted. During the process their body (and clothing, but no other items) is changed into a red mist, which expands to the shape of a dragon and then solidifies, if the mist is pierced by iron or steel, they are unmade.
As a dragon they are even stronger, can breath fire as well as “healing” fire, which is blue green in colour and heals most injuries and illnesses instantly, they may not speak, however the mind to mind communication ability makes this a non-issue.