Here is a thread with advice on command codes, both general and specific.
Kiara has a Command Code of her own, Nun of a Merciful Gaze and Warm Expression, which is a while away yet, but would improve her anti-Ruler niche. It would probably go on one of her Buster cards.
Alter Egos are middle-of-the road for stargen and Kiara has low hit counts, but crits pretty well when she can get them thanks to her passives. Hence, stargen. Crest of Humanity (ironic, that) would be good on her Quick or Arts cards, and Holy Night’s Aurora on Arts or Buster. Armament of Victory would draw those stars to a Buster to do some facecard numbers. If you plan to solo with her, White Vessel’s Command Seal or at least Cure are good for clearing inconvenient debuffs.
Parvati is lore-linked to Disillusioned Blossom’s Command Seal (any card will do). She wants damage up generally. Her stargen is great but her base damage less so, so Lady of the Red Jewels’ Command Seal would give her some crit strength to make use of those stars. (Code: Burst as well, to a lesser extent.)
I have no particular thoughts on Jalter Summer. Berserkers don’t do stargen natively very much, but her hit counts are pretty good, so stargen ccs will give her a bit of a boost, along with star weight on her Arts.