I'm really surprised that no one in this thread mentioned the possibility of Ochako being the traitor.
The first three letters of her last name are short for Urawara which means fraudulent, Uragiru means to cheat or betray, and Uragiri means betrayal.
Her character sheet literally says "she's unexpectedly brazen and frank. Proof that there's no hidden side to her, if you catch my drift.".
There are actually several pieces of evidence to her being the traitor in the Manga itself, but by a long shot, the largest piece of evidence is when Deku, Ochako, and Shigaraki were all at the mall together.
While Shigaraki is walking off, Deku says to himself "All For One... What's he after?", and Ochako swiftly turns her head at Shigaraki with a shocked face saying "Huh? Shigaraki... You... Mean...?".
This is very suspicious because All For One is a villain that's not very well known to the public and is controlling the strings underground. She's also likely surprised that Shigaraki is working for All For One because she is reporting to a level above Shigaraki.
I firmly believe she is the traitor, and the cases for many of the other characters being the traitor are a lot more trivial.