Is there somehow you can combine the names? Maybe like [name_f]Katherine[/name_f]-[name_f]Elena[/name_f] nn [name_f]Kat[/name_f] or [name_f]Elena[/name_f]-[name_f]Katherine[/name_f]
If she’s meant to be relatable and more mainstream, I would pick [name_f]Kat[/name_f].
If she’s meant to be mysterious and off-beat, I would pick [name_f]Elena[/name_f].