OK, Giratina is a Ghost/Dragon-type Pokémon was the most powerful and the largest Ghost-type among others. Which is among the legendary dragons such as Dialga and Palkia. And the current version mascot called Pokémon Platinum Version.

Giratina located at the Turnback Cave (DP) after obtained National Dex and at Distortion World (Platinum) where at place.

One is the main form in Diamond and Pearl Versions and one is the origin form in Platinum Version currently except in the Sinnoh Dex expansion.

The popular signature move is Shadow Force.

Giratina was immune to Normal, Fighting (DP) and Ground-type moves (Platinum) and it can be 2x of damage to Ghost, Dragon, Ice and Dark-type moves.

Let me also contribute: Giratina is arguably the best wall in the game, having to share its throne with Lugia.

Has anybody tried equipping the Griseous Orb in Platinum to another pokemon besides Giratina? In my experience, it can't be done. I tried equipping it to another pokemon and the game says something like "It can't hold that."

True. Griseous Orb is a Giratina-only item. However, you can use it on ANY giratina, even other version's Giratina's. Giratina is only in Origin Forme if in the Distortion World or while holding a Griseous Orb. In Origin Forme, it has Levitate rather than Pressure. It can also only learn Fly while in Origin Forme. In Origin Forme, it also has the potential for cooler spin-off fan made games, because of the whole tentacle-y clawed awesomeness.

The TM/HM moves change between forms? I guess I should have expected that, but still, a Giratina with Fly sounds cool and convenient.

It is. It can also learn Defog in both forms. BTW, has anyone else tried teaching Giratina Shadow Claw, Ominous Wind, Shadow Force, and Air Cutter? it really does work as a high-damage tank.

Thats wierd, because my girantina was able to learn HM fly in my pearl, obviusly being normal form. (please excuse my terrible spelling DX )

Normal Forme is able to learn Fly. I think Origin Forme can't. XD I know this because my original Giratina disliked me due to being my Flying slave.

# Giratina is the only dual-type Pokémon that has weaknesses to both of its own types, as well as the only Ghost-type legendary Pokémon.
# No other Pokémon has the same type combination as Giratina.
# Giratina has the highest HP stat of all legendary Pokémon, however, it is tied with non-legendaries Slaking and Drifblim and beaten by Snorlax, Wailord, Wobbuffet, Chansey, and Blissey, all non-legendary.
# Giratina is one of very few non-Flying-type Pokémon that is able to learn Fly. The others are: Vibrava, Flygon, Latios, and Latias (all four of which have the ability Levitate), Mew, Smeargle, and Arceus (all three of which can learn all HM moves). It is the only one who does not permanently keep Levitate. Additionally, before the revelation of its Origin Forme, it was unique in learning Fly but not having legitimate access to Levitate at all.
# It is one of eight Pokémon which can change forms, the other seven being Castform, Deoxys, Cherrim, Burmy, Rotom, Shaymin, and Arceus. Giratina's Origin Forme is only available in Pokémon Platinum and later game. Among all members of the Dragon trio, Giratina is the only one not having Special Attack as its highest stat, having HP as its highest stat. # In its Origin Forme, Giratina has the highest Attack stat of all Ghost-type Pokémon.
# Giratina's Altered Forme is the second heaviest of all Pokémon, surpassed only by Groudon; however, its Origin Forme is the fourth heaviest. # Giratina is the only Pokémon of its trio whose signature move is not of the Dragon-type.
# Giratina is the only one of its trio without a jewel located somewhere on its body.
# Giratina is considered to be the forgotten one among itself, Dialga and Palkia, even though all three were created by Arceus.
# Giratina is the only Pokémon that is able to hold the Griseous Orb; a Ditto that transforms into an Origin Forme Giratina during battle will immediately change back into Altered Forme (unless in battle with it in the Distortion World).
# In Platinum, if Giratina is knocked out in the Distortion World, it can be battled again in Turnback Cave (and thus in Altered Forme).

Was that from Bulbapedia?

NO.

sorry...

<_< Don't give in to teh deceit!

Yes it was...
Giratina Article: Trivia