I'll add everyone to the memberlist who hasn't been already.
Name: Olli97
Partner: Latios
Reason for Joining: I just really like the concept of the duo, with both of them being related to each other, despite not being the same kind of Pokémon, whether they are siblings or lovers. I kinda bend more to the site of them being siblings, as I just think it makes more sense, both in the movie, and in general.
I actually remember back when I first encountered Latios in my Pokémon Ruby years ago. At the time I was in such a hurry to get to the Sky Pillar to catch Rayquaza, I didn't even realized I had never seen the Pokémon before, or even how legendary its sprite looked, so I just ran away from it and went on to Rayquaza. It wasn't untill later, when I for whatever reason was checking through my Pokédex and found Latios down there at the bottom, that I realized it actually was legendary. I really felt stupid at that point >_<
But anyways, I ended up spending a lot of time searching for it, just like I had done with the roaming legendaries in my Silver version, but alas, I never caught it. I think the reason for that was that I just gave up at one point, and lost my game later. But oh well, it gave me some great memories to keep from my Pokémon games.
What would you name and how would you design a secretly new Pokemon that could be a long lost relative to the twins?
I can't actually think of anything original here, so I'm just gonna agree with what people already have suggested. The name is a rather tricky one, as it should fit into the Lati@s pattern, without sound completely stupid, so Laties would probably be the best one to go with. I also agree that the colour should be purple, as it's another good way of indicating that it has some kind of relation to the two, even if it wasn't blatantly obvious already. As for what its relation to them should be, it depends on what Latias and Latios really are to each other. If they were siblings, I suppose it would be a parent to them, and if they were lovers, it could be their child. But psshh, so many choices.