Abilities and Strength
Tarna is a quite strong Aes Sedai; she is strong enough and able to open alone a gateway to travel.
In "The Wheel of Time Companion" her level of strength is described as 19(7) which is a bit low considering the rank and role assumed by Tarna in the main books. Most probably her real level is around 17(5). Tarna's true level of strength can be guessed also by the time she spent as Novice and Accepted which was in total fifteen years, roughly the same as sisters that are around level 17(5), in fact the time spent in training usually reflects the strength of an Aes Sedai.
Tarna is 50 years old. She was born in the year 950 NE in Altara near the Andoran border and she went to the Tower in 964 NE. After spending nine years as novice and six as Accepted she was raised to the shawl in the year 979 NE.
When she was taken to the White Tower, Tarna was unable to touch saidar unless she had her eyes closed, and therefore was effectively unable to channel. Galina Casban helped her overcome that by beating her until she overcame her block. This was technically illegal, but the Mistress of Novices at the time kept silent about it.
She refused Galina's advances to be pillow friends once she attained the shawl. Upon being made Aes Sedai, she chose the Red Ajah.
After the Tower split, Tarna was made an emissary by Elaida do Avriny a'Roihan and sent to Salidar to meet with the rebel Aes Sedai there. Once there she relays Elaida's message and is told by the Little Tower that they need time to consider the 'offer'. Before leaving she meets with Elayne Trakand and Nynaeve al'Meara, trying to get them to return to the White Tower with her but they refuse.
Back to the Tower
Scared by what she sees in Salidar, Tarna sends a report back to Elaida and makes haste for the Tower. On the way to Tar Valon, she encounters a group of Asha'man in Murandy.
She arrives in Dorlan and takes command of the Aes Sedai who escaped the Battle of Dumai's Wells that are there. She is soon replaced by Katerine Alruddin. She has to sneak back into the White Tower with Katerine due to the rebel Aes Sedai siege of Tar Valon.
When she arrives she is ordered to keep quiet about the Battle of Dumai's Wells and is elected by the Hall of the Tower to be made the new Keeper of the Chronicles when Alviarin Freidhen cannot be found. Tarna meets Pevara Tazanovni in secret and they discuss the White Tower/Black Tower situations and that they must approach the Asha'man.
She is present when Beonin Marinye arrives back in the White Tower and meets with Elaida. She is one of the few that Elaida trusts, and is allowed to learn the new weaves Beonin picked up from the rebel Aes Sedai. Tarna meets with Elaida to update her on current events with what the Ajah Heads have decided to pass on. She informs Elaida of Egwene al'Vere's regular meetings with Silviana Brehon stating she must be near breaking point now.
Tarna was one of the Red sisters, along with Pevara, Javindhra Doraille, Jezrail, Melare, and Desala Nevanche, sent to the Black Tower to put the proposition of Red sisters bonding Asha'man as Warders to Mazrim Taim. He has agreed to this.
With the raising of Silviana Brehon, Tarna Feir is no longer the Keeper of the Chronicles.
Unexplained change in personality
Pevara and Tarna share a room at the Black Tower, and both have grown increasingly dissatisfied with their lack of progress. Tarna, in particular, itches to escape the Black Tower, until suddenly she disappears briefly and then no longer seems to feel any need to leave. She, like Asha'man Mezar Kurin, have suddenly changed from antagonistic towards Taim to seemingly supportive while at the same time appearing to those who know them to be in some way dead inside. Her eyes appear cold, worse than her usual distant look. Her smile looks like one "on the lips of a corpse." Pevara thinks, "Something is very, very wrong here." These are tell-tale signs that Tarna and the men were turned to the Shadow by the Turning technique.