(From the City of Heroes website)
Born Shalice Tilman, Sister Psyche is the most gifted psychic in Paragon city, and possibly the entire planet. The tremendous scope of her mental powers is far beyond that of any hero, past or present.
She has been a protector of Paragon City for many more years then her youthful appearance would imply, due to an ability she possesses called "Mindriding". This ability allows her to go from mind to mind freely while her physical body goes into a type of stasis. Since her own body is inert during this process, she does not age physically while in another's mind.
This ability has its disadvantages however. While occupying a host mind, she becomes a part of that person, and experiences the same things that they do as if those experiences are her own. Therefore, it is vital for her to escape a person's mind upon their death or she will suffer their same fate.
When the Rikti War descended, Sister Psyche knew she had to make her gift count. She did, but at a price. She exhausted her powers so thoroughly that she fell into a coma. A young heroine named Aurora Borealis offered to play host to Sister Psyche's projected mental self until Shalice's body healed. When the time came, the two minds had become so integrated that Numina needed to perform a ritual to return Sister Psyche's mind to her own body. Sister Psyche was immensely grateful to Aurora for what she did to help save her and has dedicated herself to helping to shape the tremendous gifts of the young heroine.
Sister Psyche has many psychic powers, including the ability to read minds, and is able to communicate to her teammates telepathically. This allows her to inform her team about their enemies' weaknesses as well as their identities. She can also control the minds of others, and protect herself and her teammates with mentally projected force fields.
There are two known incidents where Shalice lost control of her psychic powers, leaving her extremely vulnerable.
The first was before the war, when Shadow Queen used Sister Psyche's own abilities against her by amplifying and focusing the negative thought of others into her mind, paralyzing her.
The second time Sister Psyche found herself unable to use her powers occurred when Lord Recluse used a powerful magical force to strip the heroes of Paragon City of their powers. During this time, Shalice found herself trapped with the psychic imprint of Aurora, resulting her suffering a psychotic breakdown of sorts as she wrestled with the two personalities in her head.
Sister Psyche was able to emerge victorious in both scenarios however, and is once again fighting for a better Paragon City as part of the Freedom Phalanx; one of only 7 of this group to survive the Rikti War.
A special bond developed between Sister Psyche and another member of the Freedom Phalanx named Manticore. During an extremely dangerous mission to infiltrate Lord Recluses lair, Shalice was the only member of the team "in" on Manticore's plan to "betray" his friends and "join" Lord Recluse's villainous organization. The infiltration didn't quite go as planned, endangering Sister Psyche's life when she was used as a power source for a new giant spider-like robot. Manticore was able to save her from a grim fate, and the two then fought side by side with the other heroes of Paragon as they destroyed the army of mechanical arachnids created to overtake the city.
The two heroes later married in a very public ceremony in Ouroboros, and then protected Paragon City as Husband and Wife.
It was hinted by the Paragon Studios developers that Sister Psyche may not be dead after all and could possibly return.
Her counterpart in the Praetorian Universe is Praetor Tilman, aka Mother Mayhem.
Her counterpart in the Cimerora past was Sister Solaris.
A character resembling Sister Psyche named simply "Psyche" was seen in "Future's Guardian", set in the world of Champions Online MMO, as part of the "Psilence of Conscience" storyline. However, this turns out to be a character based on Sister Psyche created by Galatea Future as part of her fictional book series.
(See her Paragon Wiki entry for more information.)
Her death was mentioned in passing in "The Guardian Powers" #10.