Here's a random thing I thought about recently. I really like how the major/cover legendaries of each generation often have a different role in their stories/games/regions compared to the main legendaries of other generations. Some are neutral towards each other, others are close allies, while others are enemies. The role for the third legendary (when there is one) also varies.
Let’s take a look at how it has been throughout the Generations.
Mewtwo was the only major legendary of Gen 1, so it doesn't really have a relation to any other major legendaries because there were none to have a relation to. That said, it does have a bit of a connection to Mew since Mewtwo was cloned from Mew. There’s not much about this in the games apart from it being mentioned, but other media goes a bit deeper with it. Most notably the first movie, where Mewtwo and Mew end up fighting each other. In Pokémon Adventures, Mewtwo and Red are also fighting against Giovanni and Deoxys in the FR/LG chapter, though that doesn’t really count towards this since Deoxys is from Gen 3.
Lugia and Ho-Oh are mostly neutral to each other. They have never fought against each other in any Pokémon media as far as I know. In the distant past, they used to rest on top of the Brass and Tin/Bell Towers. But after the Brass Tower burned down, Lugia went to the Whirl Islands instead. They are also a duo without a master, but each of them are the master of a lower trio. Lugia is the master of the legendary birds while Ho-Oh is the master of the legendary beasts.
Groudon and Kyogre on the other hand, are in conflict with each other. Groudon wants to expand the land, while Kyogre wants to expand the sea. They fought in the past, and in the present as well. In the games, this happened in Emerald. It also happened in Pokémon Adventures. In both instances, Rayquaza had to be summoned in order to stop them from fighting.
Unlike the above, Dialga and Palkia are not enemies, but have a more neutral relation to each other. They are not in opposition, they exist together in harmony. Time and Space needs to coexist, and so does these two dragons. If they fight, it is completely unnatural, like when Cyrus forced them to battle against each other in Pokémon Adventures. Meanwhile, Giratina represents antimatter and resides in the Distortion World, which is where it was banished to for its violence. It is mostly neutral towards the other two in the games. This does not appear to be the case in the anime though. From what I have understood, all three fought with each other there? Though it seems like they also fought together against Arceus. I haven’t seen those movies so I don’t know how they were handled there.
Reshiram and Zekrom are opposites as they represent truth and ideals, respectively. They were originally one dragon, working with the twin heroes to create Unova. But the twin heroes pursued different things and the dragon split in two, each of the dragons joining one hero. They battled afterwards, but the battle became a tie as the dragons were equally strong. In the games, the player gets one dragon while N gets the other, and they get to battle each other again towards the end of B/W. Kyurem is simply the remains of the original dragon, being able to fuse with one of the others. The original dragon is never seen, sadly.
Xerneas and Yveltal are also in conflict as they are representing life and death, respectively. While this isn’t touched upon in the games all that much, it is explained deeper in Pokémon Adventures (as well as in the anime, I believe?). In Pokémon Adventures, Xerneas and Yveltal are in a never-ending conflict. When Xerneas gives life, Yveltal steals life. And so it goes on for all eternity. The only one who can stop them and create order is Zygarde. And that's exactly what it ends up doing.
Solgaleo and Lunala are basically allies (mates?), both being final evolutions of Cosmog. They coexist in harmony. Necrozma is the enemy here as it wants to steal their light (and light in general), by absorbing them to change form, and ultimately to regain its true form as Ultra Necrozma. Solgaleo/Lunala fought with the four Tapu to defeat Necrozma in the past, and Necrozma was banished from Alola afterwards.
Zacian and Zamazenta are close allies, having fought together against Eternatus in the past. In the games, they team up once again, alongside the player and Hop as well, in order to defeat Eternatus once more.
Apologies if I got anything wrong here, this post became somewhat rushed because I didn’t want to spend an eternity on it and I didn’t do as much research as I could have. A lot of this is also based on my memories from the games and manga, as well as the anime to an extent.
Either way, I really like how the roles of the major legendaries varies throughout the generations.