Signum would work. She's loyal, honest, determined, and gets on well with Fate already (in a pseudo-macho 'we'll settle this another time' sort of manner). Really, if it wasn't for the 'current' age difference, I'd say they were pretty well matched. Now fast forward 6 years to the teenage version of Fate, and that age problem starts disappearing
The real problem though... is that the Knights don't appear to age. Notice how Vita still looks like a 9 year old even in the 6 years later bit? Though I guess Hayate could change that if she really felt it necessary...
However, I'm still rooting for Nanoha x Fate... they just look really cute together, and cute counts for a lot.