I have soo many, damn.
Mita Souzou
At the time of her character's creation, I was in my first year of being an anime fan. Hence, I watched mainly popular stuff, specifically it was Fullmetal Alchemist. I created a group on Deviantart called "Elric Alchemy School" and designed a uniform, which I outfitted Mita with. Nobody ever joined the group, but Mita ended up having that as her outfit and is still, officially, a student.
Mito Souzou
He's also a student at Elric Alchemy School.
Seibetsu Shiranai
I came up with a concept for an utau with a quiet, sad and closed-sounding voice, and gray colors fit like a glove. At the time I was also learning about albinism and the hikikomori phenomenon, so I put both of those qualities on the character. A while later I created a more contrasting vocal for the same UTAU, and the design was changed from loose at-home clothing to warm "I'm actually going out" clothes. New hairstyle too, one that looked trimmed and neat as a contrast to the messy and shaggy style of the first design. Both of these designs are legitimate simultaneously, though.
Seibetsu Symmetris
The idea for this UTAU was a sort of duality thing, a bit of a Rin/Len mimic. Like Shiranai, he's also got a lot of gray, but his hair is bleached rather than naturally light. Although he strongly identifies as male, he's still very proud of being intersex, and wears a lot of graphic tees that express it. Mainly ones with fake bandages around his chest, others that have explicit jokes about his downstairs area, etc.
Bakane Gureru
An abandoned UTAU. Was a middle aged demon with a navy/maroon theme.
Kurone Raito
An abandoned UTAU. Was a young angeldemon with a black/white theme, and violet eyes.
Bureine Kiku
An abandoned UTAU. Was a first-generation demon with a neon theme.
AWG
He initially had no design, because he was just a random bank recorded mid-August. But when I decided to update him in December last year, I ended up naming his folders "aubergine" and the name stuck. So he's an eggplant.
I have more, but some are more abandoned than others, and some are completely designless (such as LEXYS or YOU). But I'm very happy with the designless ones.