Sylph of Heart versus Knight of Breath
In the event that these two battled, it all depends on the maturation level of the Sylph. If the Sylph has gotten to a point where she can access her god tier powers, she could feasibly create an army to support her (see my Sylph of Heart analysis here). However, if that is not the case, or if the battle is occurring in a place that doesn’t have things for the Sylph to animate (given that treasured possessions or dead animals/people are fairly hard to come by), the fight would easily go to the Knight. As Knights are skilled with their aspect from the beginning, even early in the game the Knight would likely be able to harness the wind in order to form gigantic windstorms or other Breath-based attacks, effectively either 1) pummeling the Sylph into submission with gusts of wind or 2) just blowing her into next week. If the Knight, for whatever reason, doesn’t have access to their powers, they still likely would have an advantage over the Sylph due to their drive and motivation.
Also, let’s be honest, I could kick your butt if it ever actually came down to a fight between the two of us.