Apparently we need to go through this again, since people seem to have forgotten or ignored the evidence. Giratina, Dialga and Palkia can be declared a trio based on a large number of similarities.
Here's a list of these similarities.
1) All three have dragon as a second type.
2) All three have blank, staring eyes.
3) All three have base happiness 0.
4) All three have base exp 1,250,000.
5) Their heights all end in 9''. Dialga- 17'9'' Giratina- 14'9'' Palkia- 13'9''
6) All three learn Dragonbreath and Scary Face, Ancient Power at 20, Dragon Claw at 30, Heal Block at 50, Earth Power at 60, Slash at 70 and Aura Sphere at 90. They all also learn similar moves at the same levels varying according to their first type. Dialga learns Metal Claw, a weak steel move, Giratina learns Ominous Wind, a weak ghost type and Palkia learns Water Pulse, a weak water move all at level 10. Dialga learns Roar of Time, Palkia learns Spacial Rend and Giratina learns Shadow force, (their "signature moves" of their first types), all learn these moves at level 40. Dialga learns Flash Cannon, Palkia learns Aqua Tail and Giratina learns Shadow Claw, (these are all medium-high damage attacks of their first type) all at level 80. These almost identical movepools are no coincidence.
7) All three can learn Draco Meteor from a move tutor. This is minor, but still a viable point. Well sort of.
8) All three have a base stat of 90, two base stats of 100, two base stats of 120, and one base stat of 150.
9) All three give 3 Effort Points in their stat whose base stat is 150.
10) All three have the similar, metallic-looking, "layered" look.
11) This is extremely minor, but Dialga in the national dex is 483, Palkia is 484, and Giratina is 487. 3+4=7. This is almost sure to be coincidental, but I thought it was interesting when I noticed it.
12) Dialga's name is based on Diamond, Palkia's name is based on Pearl, and Giratina's name is based on Girasol, a type of opal. All three have these corresponding gems inlaid in their bodies. Dialga on its chest, Palkia on its shoulders, and Giratina on its wings.
Getting back to the point, Giratina obviously relates somehow to Dialga and Palkia because of the similarities listed above. On the main site even, under the Legendaries section of the D/P page, under Giratina it says "This is the 3rd legendary Dragon to go with Dialga and Palkia." Though this is unofficial, it indicates the obvious conclusion one should draw from these similarities, that being that the three of them are counterparts. Once again, Giratina is shrouded in mystery. We don't know where it came from, or exactly what its function is. What better way to divulge this information than in its well-deserved own game? Those who would argue that Giratina isn't in the Sinnoh Dex, thus it can't be the star, aren't thinking about this argument enough. Their argument is based solely on the assumption that you would have an opportunity to catch Giratina before obtaining the National Dex. However, a plotline could easily be built that never gives us a catching opportunity until the National Dex is obtained. They could simply show scenes of Giratina is action, as was the case with Rayquaza in Emerald, however unlike in Emerald, you would not have the opportunity to catch Giratina, or in Emerald's case Rayquaza, before the Elite Four, as was the case in Emerald.
