Let's list the items found in-game along with their descriptions
here. I can't make head or tail of any of them, so I can't contribute
anything just yet. I'll try to figure a few things out later.

My (tiger/abisu) dug up what looks like either the hilt or sheath of
a sword on the arctic area that you can explore. It's called
kobitasaya (the ko is the kanji for old, bita is in hiragana, and saya
in katakana). I haven't been able to use it in combinations but it
sells for 0g.�@My japanese is horrible so i can't really give a
description (although i can replicate it in type but this hasn't let me
display japanese characters in past, if anyone wants me to email it
to them to translate)

Ok, so that was a sheath because finding the sword caused the
builder guy to combine the items into a soul calibur looking type of
sword. Using that in combining got me a durahan/ducklon.

also I went to the pyramid expedition with my (tiger/abisu) and
after 3-4 trips (it'll take that amount of time to explore every room,
it didn't seem to let me investigate previously investigated statue
things) I got 5 pieces of a hengar doll(bottom half of head, upper
half of head, eye, an arm, and the black/red power core thing).
After collecting all 5, and old man came by and combined them
into the hengar doll (kodaiheiki). I tried using it in combining but
no hengar, it predicts and gives a hare/tiger.

I got the henger doll in the doll in the pyramid, I'll go after the sword now.

It seems that obtaining the items are the minimum requirements
for unlocking the monsters.

I just purchased my first item from the mysterious merchant for
2000G, but I have absolutely no idea what it is. It's yellow, and sort
of resembles a sandwich. Or a package. It's called "<something-or-
other>item". I cannot read the two kanji characters that appear
before "item".

I tried to combine a fresh Mocchi/Mocchi and Abisu/Plant with it
and didn't get any new monster options, so it doesn't appear to be
a combo item.

Apparently, I can feed it to my monster.

You can feed just about everything to your monster, I really discourage doing it. I think as a combination item it affects stats.

Looking at my inventory after getting Kwagwar to S Rank, I
found that I had gotten two more items with exactly the same
name as the first one, with the only differences being the
appearance and/or color and descrption.

The second one I got is purple and looks more like a jagged
shard, whilst the third one look identical to the one I got from
the merchant, only it's green.

The parts that differ in the description are the katakana bits.
They read as follows:


Yellow: Nyaa

Purple: Pikushii

Green: Koropendora


Mystery solved. The items are, in fact, tablet shards of Mew,
Pixie and Colorpandora. Now we just need to figure out what
they do.

It seems that I missed something in regards to the tablet shards. It turns out that there are three different kinds of pieces; left, right and middle. I haven't actually done it yet, but it seems likely that you can put together a normal tablet from these pieces for GBA regeneration instead of an actual GBA cart. That's likely the shrine maiden's first question when shrining with that method.

Say, Lisa, could you please check if the above theory I posted last
month is on the level?

Also, now might be a good time to shed some more light on the
Gold Peach's new secrets.

I'll look into it. Time has been my main issue. I've been posting the unlocking drawings from work based on little sketches I make and take with me.

Um, she refers to the "GBA cartridge stone fragment" so, she just means GBA regeneration. There is no reference to the chips. Also, the book I have does not mention using them any way except in combining.

The chips add characteristics or abilities in combining, that seems to be about it. I can't make out some of them -the katakana is difficult, we keep going around the house trying to figure out what the words are supposed to sound like in English. A few are 'boosts speed' and such.

Ah, well, that's rather anticlimactic, now isn't it?

I wonder if the different shapes of the fragments for the same
species yield different bonuses?

yes

Consolation! Huzzah!

By the way, I'm deeply sorry if I've been rather pushy with the whole
guide info thing as of late. Since we've a wealth of wonderfully
insightful information in our--or more accurately, your--
possession waiting to be posted, I cannot help but be inquisitive.

Both boost loyalty, the silver extends lifespan some (three arrows up), the gold raises it more (four arrows up). There aren't actual numbers here.

Wait, there's a Silver Peach, too? Wow. Convenient, to be sure.

Can it be won from a tourney?

The traveling salesman guy sells it, apparently. The gold one is gotten on a venture. The catch is, the book doesn't name the venture, it lists it by page, and shows two on that page. (and no peach is on the map, just generic blue, red and yellow crystals) It's either the big tree venture or the snow one.

The Gold Peach can be found in the snowy venture on the
northernmost portion of the map as a yellow crystal. I managed to
find one with my first monster, but haven't been able to find
another since. I haven't checked after I gave mine to Minda,
though.

I found a golden peach with my very first monster. It is in the location you mention. More specifically, there is a large white tree up there with an amber crystal in front of it. It looks to me that the event is th monster picking the peach from the tree.

Also of note, the locations of the sword and sheathe in the arctic venture are -
Sheath - near the ice crack on the SW side of the center ice butte.
Sword - in front of the igloo in the NE map area.

I actually have some pretty good maps. I may be able to send scans on Sunday.

I will fix them up and post them if you can send them along. :)