GGACR/Justice/Starter

(For an explanation of notation (ie. 236H), please click here)

Core

Neutral

f.SGuardMidStartup10Recovery16Advantage-4 is your main neutral tool. It's fairly fast, has huge range, is somewhat disjointed, and is multihit so it can hit through armor or puppets. Its main vulnerabilities are instant airdash and low profile pokes, but even still, it's often difficult to punish. Use it a lot. Comboing it on reaction to either 2DGuardLowStartup11Recovery16Advantage-2 for a close knockdown with okizeme or S Michael SwordGuardMidStartup11Recovery15Advantage-5 to get fullscreen and set up nuke zoning is quite powerful, you should focus on that when starting.
5KGuardMidStartup8Recovery12Advantage-1 and 2SGuardMidStartup9Recovery16Advantage-6 are nice pokes as well. They both have slightly lower range and less active frames than f.S, however, they are faster, making them very useful normals. Notably, 2S is NOT a low. As such, the main choice between these two is 5K since it's slightly faster and much safer while it should be noted that 2S has more range, more pushback, and hits lower.
5HGuardMidStartup12Recovery24Advantage-11 works where f.S would lose. It has similar range to f.S, but hits the ground and thus beats low profile as well as preemptively anti-airing IAD. However, it has much more recovery and a bit more startup, so it's easier to counterhit or whiff punish. Use it sparingly.
6PGuardHighStartup10Recovery12Advantage-4 is your premier anti-air. It's a bit slower than most 6Ps, but lots of active frames and a large hitbox make it one of the best 6Ps in the game. Full combos out of it are possible and rather easy, it's also a great burst bait.
c.SGuardMidStartup6Recovery11Advantage-5 is a lot faster than 6P, at frame 6 startup, however, it is a proximity normal with a short activation range, it cannot be recommended as an anti-air, it can be used as one still, generally an excellent button when you're in range for it.
Michael BladeGuardMidStartup9+4Recovery46Advantage-14 is an extremely good whiff punish that goes truly fullscreen, which doubles at being a fully invincible reversal. It costs 50 tension and is punishable if close, but, if your opponent is doing something punishable, like Faust's item throw, Millia's Secret Garden, or another Justice throwing nukes, it is good to throw out and it allows you to combo further.
j.PGuardHigh/AirStartup4Recovery9Advantage- is also a nice anti-air that allows you to combo further, it's fast and leads to nice air pressure if blocked, cranking up the guard bar a little.
j.KGuardHigh/AirStartup5Recovery16Advantage- is your best air neutral tool, huge range and good speed, also combos at all ranges, it gatlings to and from j.P which allows for pressure and combos. If you land a hit, you can almost always combo into j.D, which is a very consistent routing with high damage.
j.HGuardHigh/AirStartup13Recovery24Advantage- is your main air-to-ground normal and one of the best in the game. It has a nice disjointed hitbox and hits from very high up, with excellent conversions and it doubles as a safejump if done low enough.

Finally, we come to Nuclear BlastGuardAllStartup7RecoveryTotal 51Advantage-13, AKA nuke, bomb, missile, NB, etc, Justice's main feature and the real crux of the character. It's a slow-moving, nigh-indestructible fireball that will detonate on contact with the opponent. It's biggest limitation is that you need to hold the button you throw it with for it to travel. This removes this button from your toolkit until the nuke detonates. You can manually detonate it by releasing the button. This essentially means that you will need to know how to play around with Justice's normals and how to make better use of them, because you'll often not have one or two available.

You have 5 different nuke arcs depending on button. Each one serves a different purpose in neutral.

P goes from below her straight forward, hitting low to the ground, forcing the opponent to jump or block.
K goes from above her straight forward, hitting at IAD heght.
S goes from below her in an arc that hits normal jumps, non-instant airdashes, and most double jumps.
H goes from above her in an arc from IAD height, hitting superjumps and people above you.
D goes from above her in a wide arc, hitting far superjumps and people right above you.

In most matchups, against some players, your nuke pattern generally goes P > S > K > H, but this will vary significantly based on the matchup, the range you're throwing them from, what buttons you need access to, and your opponent. Vary your nuke patterns. (I will say that you should rarely throw D nuke, except in some niche situations. It blocks the movement lane that's generally the least important and H and S nuke works jobs at that.)

Combos

From most starters, early in your play, you should focus on gatlings into 2D. All of Justice's P and K normals can, based on range, gatling into c.S, f.S, or 2S and from there into 2D, while 5H and S normals can go directly into 2D.

2D can then combo into S Michael Sword if you want a far knockdown, or be cancelled into P nuke to start zoning and pressure, however, there are some starters where this doesn't work because they start an air combo. From those, you can go into Justice's most important normal I haven't already talked about: 6HGuardMidStartup17Recovery26Advantage-9.

6H is a massive jump cancellable launcher. Because of that, you may be tempted to use it a lot in neutral, but, 6H is extremely prone to whiff punishes, slow at frame 17 startup and weak against anyone jumping, so while it leads to huge damage, I don't recommend using it in neutral. But, it's her main combo route after grounded throw, her counter special, Valkyrie ArcGuardStartup5RecoveryTotal 38Advantage-, her main reversal, Michael BladeGuardMidStartup9+4Recovery46Advantage-14, and many other things.

Justice's combo page on Dustloop currently gives a couple different suggestions for what to follow 6H with, such as, 6H jc.SD jc.S(1)[H > S Michael Sword] or [H Michael Sword > j.H] provides near-optimal damage and good choice of oki, but requires some adjustments based on spacing and match-up. An in-between option I would recommend to new players is:
6H jc.PKS(1) djc.S(1)H > S Michael Sword
This provides a good middle ground of damage while having much more lenient timing and less to keep track of in your route than the jc.SD route. j.P will make the route stable at around all ranges and j.S(1) will set them at the right height, this route still has good damage and gives you a far knockdown which allows you to zone. Variations of this route will be the majority of your combos.

However, until you get good at nuke zoning, a close knockdown and mixup is preferable in almost all matchups anyway. Speaking of...

Oki

Pressure and Okizeme

Justice typically has 2 hit options anytime she gets a close mixup, with a plethora of followups. Those main options are:

6PGuardHighStartup10Recovery12Advantage-4, a very fast overhead. At 18 frame startup on crouching opponents (and truly unreactable at 10 frames on Potemkin), it is possible but difficult to react to, and gatlings out of it can make it safe on block.
2KGuardLowStartup7Recovery10Advantage-5, a fast but fairly unremarkable low.

At first, your main grounded combo out of both 6P and 2K should be standing S (close or far depending on range) into 2D, which you can cancel into S Michael Sword and begin zoning or into a nuke for further pressure.

Aside from these two, your other main option is throw. You cannot perform a throw on a character just waking up in this game, so in oki you should do 2P then immediately throw. You can also gatling 2P into 6P or 2K to add low and overhead components to this mixup, though 2P > 6P is interruptible on block. Throw followups are discussed in the combos section.

Reversal Options

In general in +R, you should default to blocking or Faultless Defense when being pressured or waking up from a knockdown, with the use of blocking or FD depending on matchup and situation. However, Justice does have some fairly useful defensive options at her disposal, and you should know about them. These will be sorted by meter usage and overall utility.

Backdash (44): Justice's backdash is one of the best in the game. It has excellent invincibility (1-19F of a total 24F) and a good travel distance while leaving you close enough that you'll often have a punish available if it works, usually a Throw. This is generally your best meterless reversal if you need to use one.

2P: This is Justice's fastest normal at frame 4 startup. It has relatively short range, but can still interrupt a blockstring if there's a gap and stop resets.

623K: Justice's pseudo-DP. It only has invincibility in the lower body and thus shouldn't be used too much, let alone it's bad hitbox.

236P: Justice's counter. Only works against mids and highs. It has frame 5 startup so it can be used against resets or as an anti-air. Overall, its utility is highly matchup-dependent.

236D: Saperia, an invincible dash attack. The invincibilty isn't active until frame 6, so it's better used to go trough projectiles or escape the corner.

632146HS: Michael Blade, a fullscreen true reversal super. Fully invincible until frame 15. Excellent whiff punish with an easy FRC if need to be made safe.

632146S: Imperial Ray, Justice's other invincible super, but its invincibilty is limited to frames 5-7. This makes it occasionally a good option in the middle of blockstrings, but terrible on wakeup and probably her least useful reversal overall, mostly used as a combo ender.