^^^ It's not about who is "important" to a series (unless someone is THEE character of a series, like Mario, Donkey Kong, Marth, Pit, Wario, and Sonic), but about who we want to play as, who makes us want to play the game, who adds the most to smash, who would work in smash, and who "balances out" the roster in smash.
Also, Medusa really isn't that important. She doesn't appear in Kid Icarus until the end. In Uprising, she's basically the game Zant.
Besides, there are main characters (of various Nintendo IPs) with much more to work with that she'd have to get in over, specifically Little Mac, Isaac, Saki, and Shulk, as well as important characters to larger series not in (also with more to work with), like Ridley, K. Rool, Dixie Kong, and Mewtwo.
As for female characters...
DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG, DIXIE KONG!!!!!!!!!!!!!!!
Lip wouldn't be a bad character choice, either. Lyn wouldn't either, but she's got Roy's last no hope (thanks to Marth, Roy, Ike, and Krom). Daisy is BOOOOOORINNGGG!!!
Krystal... I'll confuse you guys with my opinion on Krystal. I''ll say this: I hate the character vigilantly. I love the moveset potential vigilantly. This gives me conflicting opinions of her.