First off - I do not claim to be a great Divekick player or a great Redacted player. I do, however, put an awful lot of thought into how I play. Divekick is a heavily matchup-based game, and having a general idea of how to play against a character before the match even begins is a huge help.
Secondly - If you would like to help, if you disagree with anything in this post, if you have questions, please speak up!
I can't think of everything and I don't have access to high-level competition against every character. If you have something to add to my descriptions, please let me know! I will happily consider anything, especially if it disagrees with my thinking.
Thirdly - Matchup listings will consist of three parts:
1) A rating, initially as thought of by me but very likely to change over time.
2) A gem recommendation.
3) A basic discussion of strategy. Divekick is at least half mindgames, so the best I can do is look at how various gems change the matchup and how angles and movement interact between characters.
With all that out of the way, let's begin:
Rating: 5-5. Possibly better for Red.
Gem: Dive. There's arguments to be made for the others, but Red generally gets the most out of the Dive gem.
Strategy: With the Dive gem, you can jump at least as well as Dive can. His kick is better for aggression, meaning you have to play patiently and look for him to over commit. You want to be about 1.5 to 2 Redacted widths away from him, so that you can react to his instant divekicks and follow him up and kill him if he does a full jump. Don't be shy about using the air technique if you get cornered - Dive is definitely the rushdown character in this match, and he wants to push you to the corner.
Rating: 6-4 at least. It gets even better if she doesn't use Dive gem.
Gem: Dive, without a doubt.
Strategy: It may be that I haven't played against a really good Shoals yet, but I think Red wins this match pretty handily. Against Shoals, you want to be either very close or full screen away. Shoals controls the midrange better than anyone else and Red's extremely steep kick angle means you can't really challenge her in that range. At full screen, you can jump on reaction, get above her horizontal dives, and kill her easily. Up close, you can rush her down before she gets a kick out. Be very careful up close though - her backdash is very good, and if she lures you into a bad divekick you'll be in that no-mans land where she wants you. Don't let her get above you.
Rating: 4-6. Could be worse for Red.
Strategy: Redacted has a hard time taking advantage of Jefailey's inflating head at range since her divekick angle is so steep. If he gets overly aggressive, then you should get lots of shots at his dome. Otherwise, this is a spacing and reactions matchup where Redacted has to be very careful. A fully charged dropkick controls space almost as well as Shoals does, so try to prevent that from happening (if he gets kick factor, may god have mercy on your soul). Against aggressive Jefaileys, you want to keep backing up and try to bait a bad dropkick. Against defensive play (i.e. lots of uncharged dropkicks), try to find a space go up and come down on top of him.
Rating: Varies. Usually in the 6-4 to 5-5 neighborhood.
Gem: Dive. Kenny changes from round to round and Redacted's best gem in most matchups is Dive.
Strategy: Here's the ugly truth about Kenny - he impersonates the entire cast, but because his techniques don't really suit any of the characters he's not as good as the real things. True, if he catches you with a projectile while you're on the ground or low in the air you're pretty much dead, but Redacted jumps so high that if he lets a homing projectile go you can usually get around it. The good Kennys will quickly figure out what divekick they have at the start of the round and use air projectiles to force you to a range of their choosing based on what they have that round. Do your best to avoid the projectiles, and if you must get frozen try to get frozen high in the air, at least above his effective range. Other than that, try to go for headshots - Kenny is very meter reliant and without his projectiles he's just a bad copy of someone else.
Gem: Dive, I'd say.
Strategy: A lot like Mr. N, Kick revolves around rushing you down and building kick factor if you try to stay away. You need to stay at that sweet spot where his instant air divekicks don't quite reach you and you can still react to his higher jumps. Watch out for Party Starter! Good Kicks will time it to hit just as you're landing, but even a random Party Starter can cost you the round if you're not paying attention.
Rating: 6-4. Possibly better for Redacted.
Gem: Kick. Kung jumps so low that you really don't need Dive.
Strategy: This is all about spacing and punishing mistakes. You want to be at the very edge or just outside of Kung's max divekick range. The hitbox/hurtbox interactions are completely in your favor in this match - so long as you can react to her divekicks, you should be able to punish every single one. Keep an eye on her portals, especially if there's one right behind you.
Rating: 4-6, possibly worse.
Gem: Possibly Kick, but I'm not at all confident in saying that.
Strategy: This is a huge headache. Red's biggest weakness is she can't really move forward and rush down very well. Markman's greatest advantage is his ability to control the ground with his random items, then find the Kickbox parts and dominate the air too. Red can't really get around the ground items very well, and Precision mode upkicks deal with her divekick pretty well. This match is about carefully controlled offense. If Markman finishes the Kickbox, you are screwed until the time runs out. You can punish bad divekicks on his part, but if he runs away and gets a glue in front of him there's very little you can do other than growl to build some meter (while he gets to go hunting for Kickbox parts, a trade that Markman comes out ahead in). Pretty much any item except the oil is very bad news for you, but if he happens to throw out an oil slick use the extra momentum to get closer. Be aware that he's probably looking for you to do so, and don't get too aggressive. Remember that he punishes your bad divekicks at least as well as you punish his.
Rating: 3-7. This is easily Redacted's worst matchup.
Gem: Dive, probably. I can see a case for Kick as well.
Strategy: A good Mr. N is a nightmare. He jumps and kicks a lot like you, only faster. And once he has kick factor, you are royally screwed. The one advantage you have in this fight is hitboxes - he kicks with his back leg, you use your front leg. Because of this, there should be very few trades - all things being equal, you should win most kick collisions. Spacing is critical in this match - let him get too close and it's lights out. Keep about a Redacted and a half of width between the two of you, at least. If he starts doing lots of instant air divekicks to build meter, jump a little higher and kill him.
Rating: 10-0. Redacted always wins.
Gem: I prefer Dive.
Strategy: This is a pure test of who can bait and punish better. Be patient! Because of Red's limited offensive ability, whoever moves first will usually lose.
Rating: 5-5 overall; 4-6 when he has meter, 6-4 or 7-3 when he doesn't.
Gem: Dive. You need to be able to get off the ground quickly, and faster divekicks don't help very much.
Strategy: This is all about anticipation and reactions. Any time S-Kill is in the air you only have a vague idea of where he's going to turn up. Your goal is to take control of the middle of the stage and build meter. Eventually he will have to come to you, and his divekick is actually incredibly easy to counter with Redacted's. The biggest complicating factor is meter - he builds it much faster than you, and both his techniques can end up costing you the round. Don't get too predictable on offense to avoid the parry, and make sure you're occasionally jumping back when he's in the air to avoid Trick. S-Kill can be confusing and intimidating to fight against, but Redacted deals with him pretty well.
Rating: At least 6-4. Red has a clear advantage.
Gem: Dive. Stream moves very slowly once he starts spinning, and you need to be able to get above him quickly.
Strategy: Bait and punish. Don't let him get fire feet. Keep your distance - Redacted punishes Stream's divekicks very easily, but you need to have room to recognize when he's going to attack.
Gem: Kick. This is all about rushdown. Style is viable as well.
Strategy: Rush that shit down. The Baz is only scary at long range, since he has to charge his divekick for the right angle and then wait for the lightning to come out afterwards. Watch for his techniques once he has meter. This matchup is very Kickfactor dependent - whoever gets Kickfactor is basically guaranteed a round or three, which is why Style gem is viable. The biggest danger here is getting too offensive. You want to be close, but not so close that fast lightning dashes will kill you - force him to charge his kicks for longer to get to an angle that can kill you so you have time to react and kill him.
Rating: 6-4. Red deals with Sensei pretty well.
Gem: Dive. Sensei's divekicks move slowly and you need to be able to match or exceed his jump height.
Strategy: Keep track of his stances, and space accordingly. Once he has meter to burn, be very wary of his baiting you with air switches. If he high jumps when you're close, try to move under him.
Thanks to MarkMan for contributing!