For me, the endless fascination with Ivy lies in the 'correct' presentation of the style.  Smart yet relaxed, the clothing fitting like a second skin, so that the wearer becomes almost unaware of it.  'An IL suit', says Paddy, 'never puts on a show'.  So: it's conservative, understated, possibly a little rumpled.  The 'sharpness' associated with razor-sharp creases is missing.  It is masculine and unfussy, not effeminate and fastidious like 'mod', which spends all its time worrying about a speck of dirt here, a spot of crumpling there.  This presentation is then coupled with indifference to the opinions of others and a sense of regret when it comes to their choice of clothing.  They have taken the quick and easy route and so are unfit to judge. 
Like it or not, there is a relationship with 'Sloane'. 

I'm not trying to say anything original here, only expressing secondhand thoughts, but Paddy's remark set me thinking.