Nyx

Tough choice, I could name at least 1 for each series xD
but hmm.....
Carly from 5DS. She got shafted after the Dark Signers arc due to not in-universe reasons. 
Actually just this morning I was remembering her and wondering if her Deck kept the Fortune Ladies, based on how Kiryu kept the Infernities despite not having signs of owning them before becoming a Dark Signer just like she did for her cards.

I was gonna say Rebecca Hawkins or Mieru, but I second you Sleepy on Carly. Hell, she was the playable duelist I most anticipated for Duel Links.

Divine felt like he could have been a fascinating villain, but largely got shifted due to being "outside" of the Signer/Dark Signer conflict. Plus I can't help but think that Aki's role was largely reduced to Yusei's cheerleader after Divine's death, not helped by Crow taking the spotlight from Aki.