Cetalia is described as a tall, square-faced woman in sky-blue silk with steel-gray hair in a multitude of braids. Judging by her appearance, she must have been over two hundred years old when she died.
She was strong in the One Power, by Aes Sedai standard, being as strong as women such as Leane Sharif, Merean Redhill and Saerin Asnobar. It was judged that she was stronger than Moiraine Damodred and Siuan Sanche, at least until they both reached their full potential.
This is confirmed by "The Wheel of Time Companion" where her level of strength is described as 14(2).
Being the head of the eyes-and-ears system of the Blue Ajah, Cetalia tested Siuan just after she gained the shawl; she used many puzzles in an attempt to find out if Siuan would be useful as an apprentice. Siuan passed the tests and spent her first months, and perhaps years, as an Aes Sedai being worked hard under Cetalia.
When Siuan returned from her unauthorized expedition to the Borderlands to inform Moiraine, Cetalia made sure that Siuan regretted very much taking leave.
At some point, Siuan succeeded Cetalia, or because she had reached her full potential and been able to quit the assistant's position, or more probably for the death of Cetalia; in fact we know from "The Wheel of Time Companion" that Cetalia was killed by the Black Ajah during "The Vileness" to prevent her from finding the Dragon Reborn.