I main Robin. I'll spare you the expense of having to listen to me rabble on about my whole life history leading up to why I like him, so I'll make this one brief.
Basically, I've always had some sort of fixation for self-inserts/customizable characters in RPGs. They're supposed to be the player, and you're supposed to make them resemble yourself, but I choose not to do that. Instead, I leave everything as default (and/or as canon as possible) and play as who they really are. It isn't really my own story, but rather theirs. I still get the experience of immersing myself within the world and game by adopting the character's mindset—I see myself as them, rather than seeing them as a projection of myself.
Robin was a bit different—he already had a lot of pre-determined personality and character traits for someone who was meant to be the player and nothing more. But these traits were admirable to me. And I wanted to be just like him—well-liked, intelligent, ingenious, calm, cool, and collected, charismatic, insightful, witty, and so on. He's pretty much the person I dream of being. I've mained him ever since Smash 4, and I'll continue to main him in whichever future Smash game he's in.
Sorry if this is a bit confusing—I'm not really too good in translating my thoughts and feelings to words. That's pretty much the best way I can explain my obsession of Robin for now.