Okay so, after thinking back on it, why did Helios choose Chibiusa's dream mirror to hide in? What makes her dream of desiring to grow up to be a lady like her mother more pure than other dreams, such as say Usagi's dreams? Or maybe any of the other senshi's dreams?

I think because she has very like, snow white-like, pure heart, maybe she had such dreams so beautiful dreams that he chose her...actually, sometimes i thing it has to do with her crystal o.o maybe her crystal was a bonus point for choosing her, besides, the crystal has purifying powers.

I think it's her crystal too. Maybe because she's related to both the Moon and the Earth Kingdoms, her dreams were amplified by the Silver and Golden Crystals, and that attracted Helios more than Usagi's, Mamoru's or anyone else's dreams. If Chibiusa was an average child, he might have gone to Mamoru instead.

But what makes her crystal special? It's the pink moon crystal. If anything it's a variation of the silver crystal.

I like to think of the Pink Moon Crystal as being a cross between the Silver and Golden ones, so it attracts Helios more than the Silver alone, as Sailor Moon is not directly connected to Elysium.

I always thought it had to do with her age for some reason. >_> Younger/still a child = purer dream or something like that.

Historically, children have always been seen as innocent, and pure. I think this is partially why Usagi is shown in the anime and I believe to an extent in the manga, as being child like in many ways, although I could honestly do without some of it... anyway. I think that's probably why Helios chose Chibiusa. Add in that the Pink Moon Crystal is most likely an evolution of the Silver Moon Crystal and well, it adds to the whole "pure" image.

^^^ Agreed. Very well put Addelyn.