Shemerin is plump. She never mastered the outward calmness of the Aes Sedai and so always has a look of strained anxiety in her eyes.
Strength and Abilities
Shemerin is probably quite strong in saidar because initially she was among the leading Sisters that supported Elaida's coup, and she was also in the personal council that Elaida kept, but her weak personality and behaviour were judged unfitted for an Aes Sedai by the new Amyrlin, and so Elaida decided to demote her.
This is confirmed by the "Wheel of Time Companion" where her strength in saidar is described at the level 15(3) - quite high for an Aes Sedai, and even stronger than most Sitters. However, she spent an extraordinarily long amount of time in training for her strength level - 14 years as a novice and 17 as Accepted - because of her unsuitability for testing. This is far, far longer than sisters who are much weaker than she is in the Power, sisters in the books at levels 33(21) to 37(25) spent about 20 years in training to Shemerin's 31 years. It is a testament to the Tower's patience that she was allowed to test despite her unsuitability. In fact, if she hadn't been so strong she would likely have been put out of the Tower without being tested for Accepted.
She never mastered the Aes Sedai calm, and was a complete coward. She tried to find a Warder, but stayed as close to Tar Valon as possible.
She did, however, have some strengths - she was a good Healer, a skilled administrator and organizer, and had a deft hand in internal White Tower politics.
Born in 828 NE, she went to the Tower in 845 NE. She was finally raised to the shawl in 876 NE. She refused the test for Accepted the first two times; after she passed, she hid for three days and when found was still shivering. She also took three tries to test for Aes Sedai, and barely passed the third time.
When Nynaeve was in the White Tower, Shemerin taught her about the Yellow Ajah's eyes-and-ears signs. She was not supposed to as Nynaeve was only Accepted at the time.
After Siuan is deposed, Shemerin becomes one of Elaida's advisors in her personal council. But Elaida was not glad about her counselors because they had started to take decisions without even consulting her.
During one of the council meeting, when Elaida starts to talk about the Dragon Reborn, Shemerin clasps her plump hands tightly together, behaving very nervous. When Elaida forces all of the present Aes Sedai to look at the painting of Rand on her wall, claiming that cowards are of no use to the Tower, Shemerin wrings her hands and tears well up in her eyes. Finally, as Elaida continues on about Rand, Shemerin's knees give way and she sinks to the floor. When Elaida dismisses them, Joline helps Shemerin to her feet. Elaida decides that something needs to be done about Shemerin behaviour and thinks that the sister punishment can be used as an example to the others.
When Elayne and Nynaeve tell Sheriam and the other leaders of Salidar that they have discovered that Shemerin was demoted to Accepted, they do not believe it, but while in Tel'aran'rhiod, Sheriam reads a report about Shemerin being demoted to Accepted, and then running away from the White Tower. The Sitters were aghast at this demotion, but could not figure out how to oppose the decree. Under Elaida's orders, Silviana snatched Shemerin up and bundled her off to the Accepted quarters. Shemerin was advised by other Sisters to ignore Elaida's edict and simply return to her own rooms, but she did not have the confidence to do so. Other Tower Initiates began making fun of her, and sending her to punishment to toughen her up. Silviana herself felt that Shemerin was indeed Aes Sedai, and that she should have kept her calm despite this "penance" of Elaida's, and kept being strict with her, hoping that the punishments would toughen her up. Shemerin became so nervous that she lost any Aes Sedai dignity she did have, and continued to bumble and bungle. Eventually, she broke under the strain and decided to escape.
Shemerin is spotted by Gawyn Trakand in the Rebel Aes Sedai camp upon his entrance to the camp to met with Gareth Bryne. She is passing herself off as a wash women in the camp under the assumed name Tagren, though does not seem to be a spy but merely apathetic and accepting of her diminished role as an Aes Sedai stripped of the shawl. Under questioning by the Rebel Aes Sedai she discloses her escape route out of Tar Valon, a storm drain tunnel, which allows entry to the city by a small force of soldiers during the following Seanchan raid. Shemerin found out about the tunnel because she was responsible for caring for the beggars in Tar Valon.
It is unknown whether she was promoted again to the status of Aes Sedai when the Tower reunited.