Captain Jigglypuff
*On Vacation. Go Away!*
I feel like Darkrai will be one of three Gen 4 Mythicals that will be available through normal gameplay. My reasoning is that Darkrai always felt like it was originally going to be a Legendary counterpart to Cresselia given the latter’s side quest involved the boy being trapped in a nightmare induced sleeping state and kept mumbling about Darkrai but Darkrai wasn’t quite ready and the game deadline was really close and so the developers just made it an event only Pokémon to buy themselves more time. The other two are Manaphy which I’m thinking will be given to us as an Egg during a post game side quest that is lengthy and Shaymin which I think will be used to develop Marley’s character even more as she and Mira are the two optional companions that travel with you temporarily in a certain area that had the least amount of time spent together and forming a bond. Shaymin being lost and Marley suggesting that you carry it to that White Rock would make sense and then you two walk down that long path of flowers and she has a conversation with you as you two take Shaymin home where it then wants to join you on your journey because it really likes you in the end.