Under Night In-Birth/UNIEL/Seth
Seth is the only character with an air backdash and fastfall (j.22 or j.2A+B). The air backdash does not consume the usual 1 block of GRD. His ground backdash pops him up into the air with a backwards jump unless a downwards direction is pressed before the backdash finishes.
L = Low, H = High, UNB = Unblockable, CS = Chain Shift
Startup includes the first active frame.
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214C~C > 6C route
You basically want to go into this whenever possible. The damage is good and it ends in oki. A full notation example using a midscreen 5A starter:
5A > 5B > 5C > 214C~C > 6C > j.236B > j.214A > 214B~B > [stuff] > Orb Hits > 6C > j.236B.
The [stuff] section is basically anything that hits them enough to buy time till the orb you made earlier hits. The basic easy one is 214B~B > 22 > 2C(1) > 623A > Orb Hits > j.236B. You actually have a good amount of time to link that 2C, so executionally I find this one easiest, since you can visually watch them drop and time your 2C as they fall near.
Once you get the hang of this, I personally suggest using this route midscreen: 214B~B > 22 > dash C > Orb Hits > 6C > etc. The reason I suggest this one if you can do it is because you can go into an unblockable setup extremely easily from this. The timing is a bit stricter. I haven't confirmed it, but either his 22 fastfall is dash cancellable or it just psychologically is really easy to time if you immediately input 6 6+C right after doing 22 (or 2 A+B). In any case in my personal experience, taking your time with the 214B~B > 22 link, and then executing the dash 66C quickly works much better than the other way around. Also if you want, you can cancel 6C into BC for a charge in the middle of the combo.
The third common way to fill midscreen is with 214B~B > back airdash OR delay j.214A. This route is nice because if you're bad at getting your 22 out, you don't have to worry about it here. It's a bit easier on the hands so if you're having a lot of trouble, try this route.
If you're having trouble with the 214C~C > 6C link, if you look at the opponent who's being stunned by 214C, you'll see that the initial "stunned" effect diminishes right about the time you need to input 6C. If you're having trouble with this link, try that as a visual queue. Alternatively, looking for when seth recovers from 214C~C looks viable too.
Also if you're not getting your first j.236B > j.214A link, try doing it later than you might expect.
The main issue with this 214C~C > 6C route is that it only links from certain starters. The big one is that 2A into a B then a C before 214C won't combo. You need to do 2A > 5C straight into 214C or something similar if you want it from a 2A starter. This actually influences the way you poke and do block strings if you're looking for something good from 2A (unless you're just a god at confirming off of single 2A hit Kappa). If you are combos off of 2A and you're not confident you can hit / block confirm well enough, that's when you should start looking for alternate combos.
Unblockable Route - ~214B~B > 22 > dash C > Orb Hits > 236A/B > EX Command Grab > j.2C > j.214B (Orb forces block inbetween j.2C and j.214B)
Really straight forward unblockable setup (in case you don't know, j.214B throws them even in block stun) and one of the main reason I'm going to list comboing off of j.214B > CS as an essential thing to know how to do. If you're already familiar with the dash C version of the midscreen BnB, adding this ender on is really great. Gives you ~4.5k total for using one super .
Note that if you are not close enough after dash C, you won't reach with EX command grab. If you're having issues with this, hold your 66 input for a bit before pressing C, gauging the height as they fall. You want to hit them as low to the ground as possible so that you have more time to dash and get closer to them to be in range.
Air to air stuff > j.B > j.C > j.214A > 214B~B > j.236B
Doing oki off this setup can be a bit more awkward than the 6C > j.236B route, and seems to be less prefered, but it's an extremely crucial combo route to know. If you had to many ground hits etc to do 214C~C > 6C, this is the route you want to do instead.
The main issue you may come across is that when you do j.214A too high in the air, it either becomes really obnoxious to actually special cancel into 214B, and sometimes nothing will come out. Other times, 214B~B won't combo, even when the proration etc is fine, just due to positioning. As a result, you want them to be as low as possible before doing j.214A. You can do this either by delaying your jump normals or adding extra normals in (doing j.A > j.C > j.B for example). If you're not sure whether it's going to work as an ender, remember that the ender is probably the most essential part of any Seth combo. If you don't know, try using the alternative of j.214A > land > 236A/B (for when you're unsure if 214B~B will combo) OR canceling your jump normal into j.236A/B).
j.214B > CS > Assault j.A > delay j.B/j.C > 2C(1) > 214C~C > 6C > etc
Since a lot of his gameplay runs around unblockables and the threat of unblockables, it's extremely important to know how to get damage off of landing your B dive. Comboing off of it via CS is one of the easiest ways. During CS, you have a huge buffer window, so you can input your j.6D fairly early during the CS flash and be assured that your assault will come out properly and as early as you need it to. The j.A afterwards can generally just be done early, with a big amount of delay before the j.B. You generally will want to use j.B, but use j.C on light characters. You don't need to delay much if using j.C.
Corner: Stuff > 214C~C > 6C > j.236B > j.214A > 623B into corner > 623A out of corner > 236C > (236A/B)
Really basic easy corner unblockable setup. Just watch what they do and then confirm into B dive once they block EX orb, or combo if they flailed. Can omit the 236C and just do a basic 6C ender which makes this an easy corner combo as well.
Airhit 5B > 623B routes
Note that a lot of the time, you need to slightly delay 623B. If you cancel immediately, the followup into 214A/C ~ C will whiff because they're too high (char specific I'm sure).
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