Y

 is an item used to craft the Synthesizer v2.

Obtaining
Y can be obtained by using an Atominizer on a Wai. A Wai is spawned when 2 Exes come into contact, which can happen after they are lead to each other with the Charminizer.

Synthesizer v2
Up to 30 Y can be placed in the Synthesizer v2 to gain per item.

Synthesizer v3
Up to 40 Y can be placed in the Synthesizer v3 to gain per item.

Essence Crafting
Y can be used to add Stars to Shimmering Light Armor.