Tessa became Sage in X-Men (2nd series) #109. She was referred to as Diana Fox in New Excalibur #20, and became Britannia in #22. Tessa is apparently just an alias she used as an undercover agent for Xavier.
Sage previously possessed telepathic abilities, first mentioned in X-Men (2nd series) #29, but shut them down prior to the Revolution in order to mask her thoughts from Elias Bogan. After being imprinted with Roma's god-like knowledge in X-Men: Die by the Sword #5, Sage's telepathy returned in New Exiles #5. She later began demonstrating other powers, such as flight, before ultimately merging with the Panoptichron in New Exiles #18, losing her human form. She eventually reverted back to her human state prior to appearing in X-Treme X-Men (2nd series).