Appearance and age
She is stout with a plump and round face, she has glossy black hair. As a Wise Woman she wears a very long red belt.
She is younger than Reanne (who was 412 at the moments of her death), but older than the rest of the Kin in Caemlyn, including Kirstian who is known to be around three hundred and fifteen, so Sumeko presumably is more than 350 years old. This is confirmed by "The Wheel of Time Companion" where we find that she was born in 598 NE, so she is at least 401 when she first appears.
Strength and abilities
She is described as quite strong in saidar, at the same level of some of the most influential Aes Sedai. This is confirmed by "The Wheel of Time Companion" where her strength level is described 18(6).
She is also extremely talented at Healing, using very effective weaves of her personal invention that even Nynaeve al'Meara is unfamiliar with. She had a natural feel for the nuances of Healing. In fact, although it was forbidden by the Kin's rules, Sumeko for centuries has continued working and experimenting on Healing with the One Power, and she has become very experienced and formidable with it. She was able to teach herself Nynaeve's method of five Powers healing.
Her name indicates that she is from Arafel, due to her first name ending in the common Arafellin, female -ko and her surname ending in -van. This is, however, never confirmed.
She joined the White Tower in 614 NE. After spending five years as a novice and five years as Accepted, she was tested for the shawl and suffered a breakdown during the test. She wanted so badly to do well that she pressured herself too much. She left the White Tower and eventually ended up in Ebou Dar. If she had succeeded she would have chosen the Yellow Ajah.
She discovers that someone is murdering Wise Women in Ebou Dar.
She Travels with the the rest of the group led by Nynaeve and Elayne from Ebou Dar to the Kin's farm with the Bowl of the Winds. She seems to have developed some back bone when dealing with Aes Sedai now.
With Reanne Corly's death, she has now become Eldest. After a number of Kinswomen deaths, Elayne tries to get the Kinswomen to stay together and Sumeko to enforce it. She and the Kinswomen are used to help Heal all the injured after the battle outside Caemlyn. During the siege of Caemlyn, Sumeko spends most of her time Healing .
After the siege she and Alise Tenjile meet with Elayne discussing about the future of the Kin and they reach an arrangement: the Kinswomen will live in their residence at Caemlyn for free in exchange for offering Healing and Traveling for the Crown of Andor when it needs it. But Sumeko will not join them, in fact she still intends to be an Aes Sedai and join the Yellow Ajah, so she will ask to repeat her test for the shawl.
She finally gets her revenge on the gholam in Caemlyn. She helps create a gateway for Skimming and leaves it open for Matrim Cauthon, who lures the gholam to the entrance of it and kicks it into the gateway, letting it fall for eternity.
She takes charge of Healing the survivors after Caemlyn is overrun by Trollocs. During the Last Battle Sumeko and other Kinswomen help weave gateways for messengers to move between battlefields and help transport specific armies to the war-front they are fighting on.