Im really confused about how Phantom Spd works on Karla, what is this "Combo"?

If shes faster than her enemy she does more damage.

the Phantom Speed seal adds an imaginary +10 to her speed stat only when her sword's effect calculates the spd comparison with her opponents' spd for the damage boost of the 70% difference with the two stats. So its just more of a guarantee for her sword to work. Even with her speed bane of 37, she'll have "47" spd with that seal only in terms of the sword's eff, which is pretty good.. but she will still get doubled by faster units because her actual spd is still 37