To me a high tier character is a character that can't be realistically expected to do well in tourney all by itself but only needs one secondary [not necessarily MK] to cover all his bad match-ups. It really depends though; if that one character has to be used for most of the "important" match-ups then you can't really call it a high tier match-up.
Like, realistically GW can't beat Diddy, IC, Olimar and is much better off with a second against Snake and Marth too. At that point, whatever character you use for these match-ups you can't even call it a "secondary" anymore because that list includes like the entire top 5 minus MK. Similar cases are Peach, ROB or Kirby - like GW, there's no way they will ever be high tier unless you ban like the entire top tier minus one or two characters.
I'd call Pit or Fox high tier because their two or three really bad match-ups can be covered with one secondary character. And I mean an actual "secondary" character - not pocket top tier char that's supposed to deal with all that character's garbage MUs.