Offense
Compared to other characters with very straight-forward offensive tools, Rachel can seem pretty daunting or even confusing. Her normals aside from 6A and j.C do not cover a lot of space, so spacing them properly during offense is extremely important. The method does vary depending on what character you're facing, but generally the gameplan is to safely throw out George/Pumpkin/Lobelia/Lotus to create a safe approach or control space for an opportunity to combo into knockdown which will let you summon again and attempt to repeat the process. Against most characters the emphasis is on using Rachel's projectiles and normals to control space and create bad situations, but characters with stronger neutral tools can either make this extremely difficult or shut Rachel down so you have to be more creative to find methods to summon safely.
Once you get knock down, Rachel's primary goal is to run her 50/50 with IOH j.A, but she can also do frametraps by staggering her normals/properly spacing her projectiles, and she also has easy access to fuzzyguards. Once you get a hit, you go for corner carry or 3C knockdown then repeat the process. Her offense is extremely strong in the corner where she can have multiple projectiles out to lock down a character which gives you a lot more time/opportunities to 50/50 them into another knockdown and repeat the situation.
Because a lot of her stronger offensive tools require wind, it's very important to properly manage your wind meter. For this reason, Rachel prefers combos that keep her on the ground which will allow her wind meter to recharge during her combos. Depending on the situation, you may even intentionally throw away a chance for a more damaging combo path to regain extra wind. Corner carry is also a huge priority for her, so it is also advisable to save wind for chances to knock someone into the corner with 5CC or 5CC > 632146B (Tempest Dahlia) if you have 50 meter.
Another thing to note is Rachel's damage off of random hits is not very high, so it is important to make sure to capitalize on the hits you get as much as possible. Most characters you will need to hit at least 5-6 times to kill, which is much higher than a lot of the cast. Most of your confirms will likely be off of starters like 5A/2A, occasionally 5B if you punish or space properly, 2B/3C or j.A/j.B, 6A CH, and j.B and j.C in the air. Other commons starters are Pumpkin, George, and Sword Iris.
Rachel doesn't have much of a throw game as her ground throw range is very short and she does not have a way to kara it to extend the range, but she can do tricks like airdash full screen with wind and air throw someone out of their move, or tick throw with 2A/5A and pumpkin. Air throw is also useful for people who like to jump out of the corner. The downside to her throw game is that if her throw is teched, it counts as Rachel being hit and Pumpkin/George/Lotus will disappear.
Defense
One of Rachel's main weaknesses is that she's limited in defensive options. However, she does have defensive options that are pretty unique to her character thanks to her wind mechanic. For example, when she's knocked down, she can tech over crouching characters or away from the character attacking her if she uses wind. She can also use wind while backdashing which can help her backdash away from things she would not normally be able to escape or even backdash forward and through things to make a move whiff so she can punish. Both of these options are also enhanced greatly if B Lotus is attached. Her other options are jumping/running out with wind, and normal defensive options of IB/barrier/IB Barrier/Counter Assault, etc.
If a character is being reckless or careless with their offense, you can also use 2C to guardpoint and potentially counter hit them for a combo. The weaknesses to this move, however, are pretty big so it's a risky option that should be used with care. It's very negative on block and it can be grabbed, many characters also have safe jump setups against it. Rachel can make it safe with George, Pumpkin, or 50 meter to RC. If they're being reckless with their jump ins, 6A CH is also a sizable threat. And if you read an airdash, a defensive Tempest Dahlia is a viable tool against many characters if your back is to the corner.
Another neat thing Rachel can do is if George is already out, you can wind during the gaps of the attacking character's blockstring to move him towards you. However, keep in mind that many characters will simply jump cancel their moves to jump over George or gatling into throw to make him go away.
Neutral
Neutral with Rachel can be really difficult to understand, especially for beginners. Generally you want to start with being familiar with the distances her projectiles can move with and without wind. The idea is to setup and position your summons to either keep people out, help Rachel get in, or stall for time to get more summons out. What you do will greatly depend on what match up you're playing. For example, the idea with large and slow characters like Tager is to zone them out or force Tager to block something that will allow you to start your offense safely. Whereas against characters like V-13 and Lambda, the idea is to get in as quickly and as safely as possible.
One thing to keep in mind is that Rachel's summons are all slow, so you have to get very familiar with safe distances and spacings to get out summons. With some characters this will be greatly easier than others. For example, Tager does not have a run so you can summon safely full screen and unless he has Spark Bolt ready or you are magnetized he cannot stop you. For other characters who also have strong zoning and neutral tools, generally you will spend the match trying to find blind spots to either approach quickly and hit them into a knock down situation or to summon pumpkin to create a safe approach.
With a lot of other characters, you generally can manipulate your projectiles and wind to safely setup as long as you are reasonably careful. For example, if you miss a B Lobelia cannon many characters are fast enough to run forward past it and punish Rachel while she is still in recovery. Where as if you get them to block it that will in most cases give you enough time to set out another summon.
Other useful tools are 6B to stop characters from recklessly approach you, though it's range and speed is not the best so a lot of times you must stick it out preemptively. 236A with wind can also cover a lot of space. If a character is above you, you can stop them with air throw or j.C. If Pumpkin is out, winding that will halt the approach of most characters. Similarly, the boosted 5D effect with Lotus can also make neutral difficult for characters to deal with. And for some characters you can additionally stop them in their tracks by igniting Lotus with Sword Iris. However, the tracking is really bad so this has limited uses.