Preserving Aeris as a party member, without save-editing

Spoilers below, obviously. This game is over 20 years old.

In Final Fantasy VII, the ability to resurrect Aeris or retain her into Disc 2 is one of the long-term myths of gaming. In the PS1 version of FF7, it was possible only via the Debug Room or save hacking. In the PC and mobile versions, a major bug was introduced, allowing Aeris to be used past her 'death' without having to resort to direct hacks.

The Yuffie Warp[finalfantasy.fandom.com] is an incredibly powerful tool for breaking scripts, and can be used to retain Aeris by warping past the Temple of the Ancients. This simpler method is briefly described under TL;DR.

Unfortunately, the simple route meant missing out on unique items, including Aeris' own ultimate weapon and the Morph materia. The rest of this guide provides a full method for having your cake and eating it, keeping both Aeris and all the unique Temple items. Unfortunately, the method requires a lot of setup and 3-4 warps; if someone has a simpler way I'll gladly adopt that instead.

If you just want Aeris in your party and don't care how, editing your game save is a much more practical method of retaining/enabling Aeris. This method exists mostly as I was attempting a playthrough preserving Aeris as a party member without doing so, but was frustrated by missing out on Morph and other useful items. This guide exists because I was surprised not to find the possibility mentioned in Steam guides.

Yuffie Warp until the game cries.

Retaining Aeris without completing the Temple of the Ancients (much easier, misses many items):
Warp from before the Gold Saucer revisit to the Coral Valley and continue the plot.

'Resurrecting' Aeris while gaining the items from the Temple:
See 'Prerequisites' for save slots used.
Before revisiting the Gold Saucer for the date scenes, warp into the Temple of the Ancients and complete it. Optionally warp into the Temple again to get the Ribbon. Get the Lunar Harp. Warp to the Dyne fight (or your choice of script) to add Aeris to the party. Warp to the Coral Valley, double back and awaken the Sleeping Forest to continue gameplay.

The game should be played up to obtaining the Lunar Harp, without recruiting Yuffie. This is considered Slot 4 below. Remember to make saves at the other points below (ordered chronologically). Note that the third save requires completing the Temple of the Ancients maze room without picking up any of the ground items, so that these can be collected after warping.

The saves are not strictly necessary, but the walkthrough will assume they exist, as this method would be extremely time-consuming otherwise. The saves do not need to be from the same playthrough.

Saves

The full route consists of 4 steps, involving one warp each.

Load from Slot 3. Return to the Temple maze area (left exit). Do not pick up any items yet. Start a random battle, and get a Game Over.

Load from Slot 2. Go to any Mystery Ninja forest (Rocket Town forests have the highest encounter chance). Fight until you face the Mystery Ninja, then defeat her. (Yuffie Warp 1)

You will now be in the Temple of the Ancients, without Aeris locked into the party. Do not leave and reenter the maze area, even to use the shop/save point, as the game will softlock. Pick up all the items in the maze except the Ribbon, which is not obtainable yet. When you have all the items, continue through the rolling boulder room, obtaining Morph, to get to the clock room. Saving in Slot 5 is recommended to allow easier recovery from softlock.

In the clock room, enter each room to get the rare items (Princess Guard: IIII, Trumpet Shell: VII, Megalixir: VIII). Do not enter the Ribbon area (V), or fall down the pit, as the game will softlock when reentering the maze and you will have to restart (reloading from Slot 5 if present). Continue into the chase room (VI).

Complete the chase section, and follow the scripted events through to completing the Temple of the Ancients, remembering to pick up the Bahamut materia. You need not remove Aeris' weapon or armour before she leaves, as it will still be equipped when she rejoins the party.

After the cutscenes and waking up in Gongaga, Aeris has been removed from the party and her materia and accessory are unequipped. Don't worry, this is normal.

Take the Tiny Bronco to the Temple of the Ancients. Save this game as Slot 5.

Load from Slot 3. Enter the clock room (right exit), then the Ribbon area (V). Start a random battle inside this area, and get a Game Over. If you accidentally open the chest, don't worry; despite appearances, you will still be able to open it after warping.

Load from Slot 5. Encounter and defeat the Mystery Ninja. (Yuffie Warp 2)

You will now be next to the Ribbon chest. Open it, then go back to the clock room. Go directly to the exit (XII), and leave by the now-open door. When outside, move toward the bottom-left of the screen, running down the outside of the non-existent pyramid (this looks strange, but works). At the bottom you will walk over the trigger to enter the pit area; ignore this and leave to the left.

Note that Aeris can actually be returned to the party at later points in the game providing the Temple of the Ancients was completed by warp, without visiting the Gold Saucer (1), but that is outside the scope of this walkthrough.

Go to Bone Village and dig up the Lunar Harp for later. Go to the Gongaga or Nibelheim forests, then save in Slot 5.

Load from Slot 1. Ensure Aeris is in the party. Continue the plot until the Dyne boss fight, and get a Game Over.

Load from Slot 5. Encounter and defeat the Mystery Ninja. (Yuffie Warp 3)

You will now be in the Dyne fight with a full party. Beat him, and play through until you get the buggy. Aeris will be added to the party. Swap her out of the active party for now. Go to any Mystery Ninja forest, then save in Slot 5.

Load from Slot 4. Continue through the Sleeping Forest to the Coral Valley. Start a random battle, and get a Game Over.

Load from Slot 5. Encounter and defeat the Mystery Ninja. (Yuffie Warp 4)

You will now be in the Coral Valley with Aeris available in PHS. Head back to the Sleeping Forest until it awakens again. You can now progress with the plotline, or head back to Bone Village to dig for the other items if you wish. Be aware that having Aeris in the active party during some following events may softlock the game.

If you wish to hide Aeris from PHS again later, warping into the Demon Wall fight should continue into the Temple exit scripts and remove her. Be aware that this will reset the PPV.

There are some issues to bear in mind with this route. No unique non-Key items are missed, but 100% completion is likely prevented, depending on your point of view.

You will not receive the Keystone. It is the only unique item that cannot be obtained via this route, but it is considered a 'key item' and serves no other purpose after the Temple.

You can't get the Protect Vest or Choco Feather from Dio in the Gold Saucer without PHS-locking Aeris, or the Nail Bat or Silver Rifle from the Temple of the Ancients without softlocking the game. All are obtainable elsewhere in the game.

You lose access to the Tiny Bronco after the Sleeping Forest, and so can't return to the main continents until receiving the Highwind. Until that point, no new areas become available, and the Tiny Bronco is then removed anyway.

Some Fort Condor battles will be missed. There is no penalty for this besides missing the rewards, and if 7 battles have been fought before this point, all later rewards are only healing items. Most playthroughs would skip these.

If Cloud is not in the party the first time you visit the Temple entrance bridge, the game will softlock. This can probably be worked around by triggering the script initially before warping into the Temple, but this is unnecessary as there is no reason to return.

Some scripts break when Aeris is in the active party, as they do not set animations for her. She should be completely usable otherwise, and as we ensure PHS works correctly for her, she can be swapped out of the party before any problematic scenes.

'Saving' Aeris is not actually very difficult - a single Yuffie Warp will do it, as has previously been demonstrated. Getting the items from the Temple of the Ancients (including Aeris' ultimate weapon, 3 unique materia, and a rare Ribbon), while ensuring Aeris has the correct PHS flags, and avoiding breaking scripting, are the primary difficulties.

Unfortunately, after the date scene, leaving for the Temple of the Ancients PHS-locks Aeris permanently. She then cannot be selected at all in PHS or party selection, whether in or out of the active party. Because she leaves the party after the Temple anyway, this flag is never removed! Aeris breaks some later scripts when in the active party, so this is a major problem. We therefore have to skip the Gold Saucer and obtaining the Keystone, and so have to Yuffie Warp into the Temple of the Ancients if we want its items. The Gold Saucer fortunately has no other unique items at this point; we only miss the Keystone.

Glitching into the Temple has its own issue. A script runs the first time you enter the maze (usually only possible via the Keystone entrance). The script breaks if you enter the maze by any other entrance, as characters can't reach their animated positions. This means we can only ever enter the maze area by warp or the game will softlock, so the Nail Bat and Silver Rifle are unobtainable (both areas exit into the maze). Thankfully there's a workaround for getting the Ribbon. Because it's in a separate part of the maze area, we have to warp into that area separately afterwards. Re-entering the Temple via warp after its destruction works fine and doesn't break scripts. The door behind the Demon Wall fight even conveniently leads outside - albeit floating in the air where the building's entrance used to be! Given you never enter this door in a normal game, perhaps at one point in development the exit was meant to be usable outside of cutscenes.

On the first visit, the only way out of the Temple of the Ancients requires triggering the Black Materia cutscenes and so losing Aeris. Mechanically, she 'dies' after the final cutscene, having her accessory and materia removed, and being hidden from PHS. She effectively becomes an NPC. The later cutscene in which she narratively dies makes no mechanical changes.

Because we skipped her being PHS-locked, 'resurrecting' Aeris is fairly simple - simply warp into any battle before a script PHS-unhides Aeris. Here we use the Dyne boss fight. Barret fights this battle alone, so the player's previous party is saved, and after the following sequence of events, party selection defaults back to those party members. Therefore, if the party contains Aeris before starting the Dyne battle, she will be forced back into the party afterwards. Warping into this fight then becomes a reliable (if roundabout) way of readding her to the party. (The Hundred Gunner/Heli Gunner fight also works, but requires repeating an even longer section of gameplay and leaves the player a long way from any forests.)

The 'resurrection' causes one more issue. Warping into old scripts generally resets the game's plot progression variable (PPV). Therefore, we can't just go through Bone Village to continue the plot. The incorrect PPV leaves the following scripts in an inconsistent state, and the game will just show the Sleeping Forest/Gongaga cutscene and remove Aeris again.

So finally, we're forced to warp past the Sleeping Forest, as one would to 'save' Aeris if forgoing the Temple of the Ancients items. Unfortunately, there is no Mystery Ninja fight on the northern continent, so the warp is one-way and we have to leave the Tiny Bronco behind. This isn't a huge issue as there's not much reason to go back until you have the Highwind anyway. Awakening the forest conveniently sets the PPV to the correct point again, making it safe to progress. Scripts from this point assume Aeris is already PHS-hidden, so she will not be removed from the party again.

I've tried to find ways to simplify this route as much as possible, but it's still unfortunately pretty complex and requires setup. If there's a better method that doesn't involve hacking the game or your save files, please let me know! The following would improve this guide specifically:

A battle, revisitable by warp, which PHS-unlocks Aeris. This would remove the requirement for warping into the Temple. The Gold Saucer and date scenes would then be viewable in this method, the Keystone and Dio's items could be obtained, and the Ribbon workaround would be unnecessary. Additionally, the method could then be adapted to restore Aeris to the party on an existing save after the Temple.
A battle, revisitable by warp, which PHS-unhides Aeris without resetting the PPV. This would remove the requirement for warping past the Sleeping Forest.
A way to escape the Temple of the Ancients without triggering the exit cutscenes. This would allow getting at least some items there without losing Aeris, potentially avoiding 2 warps and much of the difficulty of the process.
A way to disable or bypass the Temple maze entrance script. This would make obtaining the Ribbon and other clock room items much easier, and would allow adding Aeris back into the party by means of the shop/save room script.