Nuts to this whole plot relevance and theories, just bring Shinki back at once. Geez... it shouldn't be THAT hard.
1) You make her sprite.
2) You code her.
3) You make her dialogues.
Ta-Da~!
1) you make the sprites for the attack animations (for which concept art is much likely needed) and special effects
2) you code the hitboxes for said attack animations, startup/active/recovery frames for them, states for the characters (airborne, knocked down, air teching, blocking high/low/airblocking, taking a hit, etc), how moves should behave ("this grab can catch teching enemies, but not enemies in hitstun, downed opponents, etc", "this attack is air unblockable", "this backdash has invulnerability in the first n frames", "this move has upper/lower/full body invincibility", "the air version of this move acts differently from its grounded version", "you can cancel this attack with a highjump/special/super when the move hits the opponent", etc), then you playtest to adjust characters' balance and check for bugs/glitches/infinite combos or otherwise broken stuff. (and also take care of code optimization; also, the character's gameplay must be thought and decided beforehand)
3) this may be the less difficult step :p (depending on how many routes/general win quotes/special condition win quotes you want to add. Story-wise, additional artwork may be needed to go along with said quotes)