It's like pets: I love you all the same, but different! ;)
Nin/Fury will always be the "main" regardless of what toons I'm playing. She's the one I can play in my sleep, for the most part, and still be half decent.
Otherwise the "secondary" is a 70 monk I levelled for fun and enjoyment, that ended up being a raid alt I bring out to MA or pull or use when we're healer heavy. She's by far the most fun to play, the Brig is the most gratifying (debuffing the bejesus out of the mob then watching it just
liquify is the coolest thing ever).
Now I'm starting to fall into the trap of "what can the raid use" alt levelling.
Our Brig is afk getting married and buying a house... mine is 69 and nearly gtg to fill in until we have one full time.
Meanwhile the SK is sitting at 59, with a pile of good basic raid gear (leftovers) & masters waiting for her in the bank...
And then I think... we really need a dirge... !
You see how it goes...
Disclaimer: I'm a complete nerd, so grain of salt here...
All my toons that I've kept have their own quirks and character to them - and each seems to suit or magnify different moods/personality traits (they ARE avatars, after all). People seem to like one over the other, which entertains me, but at the same time, I get it. Fala (monk) and Nin are the most distinctive of the two, and both are fairly different in their own ways. Possibly due to the roles they play, possibly due to what I feel they are. Not sure.
From the observation PoV, I've noticed my friend's personalities shift depending on what alts they're on, as well - one friend is somewhat passive/deferring on one toon, irritable on another, troublemaker on the next. I'm not sure if it is class, or race or what - the guys who've gone from human/elf/barbarian to gnome seem to take on the gnomish quirky/crankiness. A guildie who had a human monk had the flirty machismo thing going on the monk, then a reclusive, quirky irascibility on his gnome mage. Likewise, I've seen people who stir up shit on one toon, tone it down on another.
A friend's Barbarian monk has been an older brother figure to my "younger" monk, but his ranger and my brig are closer in "age" - it is just wierd. Or I am wierd. Or both! ;D
So that is to say that they all have their own "weight" personality wise - I'm not sure one is less/more than the others, though Nin feels closer to my 'real' skin than the rest, so she'll always be the "real" main, the rest are always going to be secondary to her, as much as I like them.