I went to the Aquarius temple first, so I didn’t understand why Longinus said to Jun “you don’t know who this seat is intended for”. The second temple I did was Scorpio, and its boss is Shadow Michele. Turns out Persona 4 just repeats the concept of shadows from Persona 2.
Michele’s plotline is quite touching, though, albeit simple: he was a fat kid, so he liked the only girl that threated him well.
When they grew up, he became fit, and she got fat, so she hid her identity (they are all using nicknames anyway, Michele is Ekichi’s nickname), then sold her soul to Joker in order to be beautiful.