What to do with - speed + red Karla?

i got the same ivs, and I gave her life and death, which helps, but makes her very easy to kill.

Karla is far from useless even with -spd dude, 36/37 is still nothing to scoff at.
Given her low defense, I would say using desperation for B skill would be a must, and she does this very effectively with her low defense and high offensive spread(yes, even with -spd)
And you needn't worry about waiting for a better one to inherit skills, all skills, learned or not are transferred over when a unit is merged.