Tonilia is a member of the Thieves Guild. She is the first fence players in the guild will have access to. She will later give a task to ship some moon sugar to a travelling Khajiit Caravan, who also become fences.
At some point, you will be sent to her to upgrade a piece of your guild armor. There is no option to improve a second piece so choose wisely, as it’s a much longer road to receive the Guild Master armor.