As far as characters I know for sure I didn’t recognize goes.
64: Link, Samus, Ness, Captain Falcon
Melee: Ice Climbers, Shiek, Zelda, Marth, Ganondorf, Roy, Mr. Game & Watch
Brawl: Lucas
After Melee I’d played every Nintendo game I could get my hands on.
Some things worth noting.
I was only familiar with Pit due to his trophy in Melee.
I only knew about Fox and Falco because I had played Starfox 64 at a Blockbuster once, and I’m pretty sure I knew of Kirby before Smash, I think at the very least I saw Kirby on the boxart of some Kirby games, probably at the same Blockbuster

.
As for Young Link I got the GameCube when it was bundled with the Zelda: Collector's Edition, I think I played Ocarina of Time before unlocking him in Melee, though I’m not confident in that being the case.
As for R.O.B., I technically knew about them thanks to F-Zero GX, I also learned more about them through the internet, though I’m not sure if that was before or after they were revealed as enemies on the Smash Bros. Brawl Dojo.
Shout to my dad for letting me play Metal Gear Solid 2 with him, I wouldn’t have not about Snake if he hadn’t.