I see 2 and 4 as spin-offs, because 3 is like the start of it all as a prequal and 1 continues that story while 2 and 4 are happening in two completely different and inconsistent places.
Final Fantasy X-2 happens after Final Fantasy X and it continues the story from where X left off. Yuna and Rikku were main characters in both games with the new addition of Paine, but it keeps with the storyline.
Like Megaman is the main series, Megaman X is a spin-off that turned into a new series of its own, then Megaman Legends, Megaman.EXE and Megaman Zero. However, though Megaman X and Megaman Zero are spin-offs they are STILL part of the main storyline while Megaman Legends and Megaman.EXE are completely unrelated to the main storyline, like this new Devil May Cry is completely unrelated to the storyline of Devil May Cry 1-4, but it is still a Devil May Cry game, just like Megaman Legends and .EXE are still Megaman games!
I know this looks like a big mess of an explanation, but I hope you can at least get a glimpse of what I am trying to say with this. XD
Nero is a new main character.