Before Suikoden 2 JP was officially released in 17/12/1998, there was a demo version that was packaged along with Metal Gear solid in 3/09/1998. In North America there was also a EN demo version when released with Vandal Hearts 2 in 30/11/1999, but that was after the official EN release in 31/08/1999. This makes the JP demo version the earliest possible version of the game in retail by about 3 months.
The game file is also much smaller at 220MB while the EN demo is about 501MB (a tad bit more than the completed full game). Since it was only a demo , a lot of stuff were unpolished since the game was never at it's complete state, but at least exploring it would give a little insight on how the game was supposedly meant to be.
Compared to both JP and EN demos, both are similar as you were only meant to play through until the Mist monster and the Battle mode. You can use the wiki http://suikosource.com/games/gs2/indepth/demo.php to get an idea but some quick difference in JP demos are:
1) Some NPCs uses different sprites.
2) The dogs are puppies and the 2 cats in Toto village uses 1 color (Normal game have 2 different colors)
3) The enemies are slightly harder.
4)The bridge at Kuskus Town doesn't exist.
5) In the Battle mode the JP demo only have half the roster available (Unavailable: Killey, Siera, Genshu, Wakaba, L.C.CChan, and Sigfiend).
6) Battle animation looks left polished in comparison.
7) It's in Japanese. duh.
Other than that there isn't much difference. In the EN demo you can bypass the Mist monster and proceed as normal and (from my experience), you can only reach up to the point you were about to receive the talisman at Muse when Pilikia asks you to buy it for her before the game crashes; the game does contains all the data of the completed game (but some are in Spanish). In the JP demo, you can also skip the mist monster.
Bypassing it and out to the world map any enemy encounter will cause crashes. But once you reach Kyaro Town you can explore and is in a near complete state.
https://youtu.be/XqVzE1iVw3w
A lot of stuff are missing and colors are mixed but that's pretty much it. It's also good to know the top right table at the inn wasn't hiding anything as in the completed game there is no collision there. You can try saving but all you get is Hix's sprite sheet. The town is mostly complete most likely because it was used in the opening scene a lot.
You can trigger Jowy and family and Nanami scenes but since it being a demo some of their action isn't present.
https://youtu.be/MfP5h1HcZL0
The main take away here is Jowy's brother uses a younger child model. That is pretty much more impactful since he looks older in the completed version when he disown you.
https://youtu.be/MqSV-Wnw_8c
Nanami's encounter is pretty much the same but less dramatic. The game crashes just before Nanami joins you so you can't progress any further.
And don't bother trying to reach Tenzan Pass since the warp point isn't working.
All Characters
Surprisingly enough you can use the enable all character gameshark code when you go into the Battle Mode at Leona.
Some characters have their own portrait or some uses a placeholder with a 'sir' or their name text. In combat it can be a mix of using their actual character or uses another as a placeholder. The squirrels are funny as all the developers did was change their color and stamp a number on their head; in combat they all uses the same Mukumuku sprites (unite attack crashes the game).
Runes
Normally you can only have Fire, Water, Wind, and Pale Gate rune; there are others but they don't level up. By hacking to enable some and maximizing your stats, the runes can become available. There are some that has less spells than normal even with max stats, like Pale Gate Rune for example, and some just crashes the game when attempting to cast:
Despite Bright Shield and Black Sword Rune spells will crash the game, in the game files there already is the FMV for their last spells.
I'll only post the ones that does work since they look different than the end product.
Fire Rage Rune
https://youtu.be/iHNSvndA8p0
Dancing Flame looks less polished.
Water Flowing Rune
https://youtu.be/kOFw9KrlgzQ
Wind Cyclone Rune
https://youtu.be/qGBXKVrylZE
https://youtu.be/3se18kyyjPA
The last 2 spells are funny when it's incomplete.
Misc
By messing with the Talisman and changing the value through Cheatengine, I was able to warp to certain town and areas. Unfortunately most of them just loads the town and nothing can be interacted with, crashes upon entering, and in one instant loading into Cave of the Wind entrance just loads the areas sprite sheet. There is one place however that pique my interest:
https://youtu.be/l3sSmi2G2Y8
It's easier to just make a video about it. I do not know what's the purpose and it seems too organized to be a glitch or whatever. I tried looking for it in the completed version but wasn't able to find it since it seems to be gone. The best I can guess is it was a very very early debug room that was used during development and was deleted soon after as the area looks similar to FF7 Debug room.
00B7CA1F Byte 06
00B7CA20 Byte 19
(Previously it was (06,19) but found out it was suppose to be (0B, 00). The previous values still works.)
M_EAT.STR ?
https://youtu.be/WPycKcKomwI
In the game files there is a FMV listed as M_EAT.STR. Going by the next entry which is M_FIR.STR (for Final Flame opening FMV (Fire rune) that you can see in the Fire Rage Rune video), it's suggested to be for a Rune for either Earth Rune or Soul Eater Rune (Earth is more likely based on how it looks).
In the JP completed version and all version to the latest PSP version (2010) it has it listed as ROCK.STR. It was thought to be for the Sajah Rock opening but looking at this it might not be the case. Omnigamer did suggest it was a FMV for a different game when in development but there doesn't seem to be any thing to suggest that.
That's pretty much all there is. If there's anything I want to edit or add I'll do it later.