Resident Evil

This is how I unlocked the achievement and I only played as JILL VALENTINE so far:

1. Normal difficulty playthrough ("Like climbing a mountain" from the main menu)
2. Normal+ playthrough (New Game Plus) ("Once Again..." from the main menu)
3. Real Survival Mode playthrough
4. Invisible Enemy Mode playthrough

Everything of course on the same save-file and that's also the order I played the game.

Additional notes:

-Both RESIDENT EVIL 5 - BSAA outfits for each character are available from the beginning.

-Complete the game once with Jill on any difficulty to unlock Jill's ARMY outfit.
-Complete the game a second time (same save-file NG+) with her to unlock Jill's RESIDENT EVIL 3 outfit.

-*Complete the game once with Chris on any difficulty to unlock Chris' THE MEXICAN outfit.
-*Complete the game a second time (same save-file NG+) with him to unlock Chris' ALTERNATE S.T.A.R.S. uniform.

*As I stated on the top that's how I unlocked them, never played with Chris Redfield. So either play it four times with Jill Valentine or twice with her on twice with Chris. Just make sure IT'S ALWAYS ON THE SAME SAVE-FILE!!!

If you guys find a faster/different way to unlock this let me know. Hope this helps!

Load status: It is required to Load the relative Cleared Data. But after completion of a story that is automatically loaded. When the software was abandoned, after that nothing relative is loaded automatically upon starting software runtime.

Once again.../New game: The, Once again...-option, temporarily replaces the, New game-option, as long as the runtime session recognizes (when loaded/preloaded) a Cleared Data, and it serves not as a game play affected New Game Plus feature beyond the use of costumes. You will be able to select a difficulty level and play a story in base mode once again.

Costume usage: The initially used costume can be selected when starting a story, and changed at the Closet by the Art Room, which is initially accessible from the lower level of the Main Hall, after the retrieval of the Closet Key from the Medical Storage Room's Item Box.

Closet Key: The Closet Key will only be in the world, as long as progression originated from a Cleared Data. It is a story completion reward for saving Chris and Barry in Jill's playable story, and/or for saving Rebecca and Jill in Chris' playable story. This item is not required for the relative progression.

Progression: In order to continue making process on getting all costumes and all weapons, it is required to develop progress further from the most advanced Cleared Data. So, always load it when starting a new story, if is not already preloaded after a story completion during current runtime session. Only then, select either, Once again..., Real Survival mode, Invisible Enemy mode.

Progress tracking: 0%/20%/40%/60%/100% (80%-100% automatically calculated, based on two alternative costumes that come within software version)

Task: After the completion of four stories in base modes of any difficulty level or any challenge modes, all costumes (four unlockable) become unlocked for the extensive progression (developed from three most advanced Cleared Datas) of a Save Data. Save the forth Cleared Data, in order to keep relative progression (forth costume/flagged Weapons' index).

Unlock order: If the character played within a story had yet a locked custom, then one will be unlocked for them, otherwise one still locked for the other character would become unlocked.

I hope now everyone of the future will better understand.

If you made other conclusions and revealed any minor factual errors in this information, please share your findings in great detail by posting a comment to this Solution.