Damn those are some tough costumers.....
With Rachel just watch out for her Roll through, its possible that the problem may not be Rachel at all but whatever talented player is Wielding her.
As Kokoro you won't be moving around alot so going up against The Clones is going to be tricky, Alpha needs her space, so you gotta bum rush her and press the advantage, of course its not that simple, she'l be waiting for you and most of the time will just smack you away everytime you get close.... you need to bum rush with a Gimmick, I like to run in and as soon as Alpha flinches I side step and counter hit, sometimes I have to do more than one side step because her linear strings retrack rather quickly. If I have a super jumping character like Ayane then I just break out that summer salt kick, or I just out right jump over her and sweep her from behind. Shes also weak to offensive holds alil bit, hope Kokoro has one.
As Kokroro you could use her 1 P+K, this will make her step back, if Alpha misses her jab as she leans in then Kokoro's tackle should snap forward and send he flying across the screen, after this you want chase after her before she recovers and corner her, Alpha doesn't do so well up against a corner. Alpha players tend to panick rather quicly if you keep knocking them down since they got no wake up kicks.
Now against Phase 4...... well you're on own....... all I can say is Phase 4 players like to forego the process of building up Critical Level in favour of her launch throw into juggle, this makes side stepping less usefull.
Sarah is a very "pokey" character, she doesn't have alot of stun potential so shes going to bait into attacking out of disadvantage to get her stuns or bait a hold and throw you for high damage, you might have to play defensively a lil bit, shes fast and most of her punches and kicks are not only safe but they are on + too. Don't try to attack in between her pokes, she'l counter hit you and get the stun she needs. decent tactic is to crush her high attack in order to get her to complete her strings then when you're finaly at advantage you counter attack, try to go for guaranteed damage set ups.
Secondly, be sure to have your fuzzy guard ready because when she notices her pokes aren't getting through she'l start going for riskier low attack and thows, these are great opportunities for you to strike back and again try to go for guarenteed damage, you never know just how many times you can keep this trick up so make each hit count. hope fully you'l get lucky and she'l hop into her Flamingo stance, her flamingo moves are not as safe as her normal moves so try to take advantage of that. you'l notice I didn't mention Kokoro for Sarah, thats because very few characters out match her by any significant margin, you'l need to mold what ever character you're using to fit this basic strategy.
And thats all the experience I can share, I still struggle with the clones but Sarah and Thunder Thighs are no problem for me.
Kokoro has great Mix Ups, don't back away, you'l want to stay close to your enemies and to do that you're going to have to get better at blocking or use the side step more often, when you sense your opponent is trying to get away from you, thats when you know you gott'em on the ropes.
I think Kokoro crouches when she performs her deceptively fast 2 H+K. Its nice ace to have up your sleeve.