Katisha was plump and matronly Nord who lived in Riften in late Second Era. She was married to a smith, had two teenage children, and was perfectly ordinary and respectable — except that she was very fond of cats (and by logical inference, their humanoid counterparts the Khajiit), had a talent for certain kinds of magic, and cultivated rather odd friends.
She was introduced to Barenziah by Therris. Later Barenziah introduced her to Straw. She taught Barenziah an invisibility spell and schooled her in other forms of stealth and disguise. Katisha mingled magical and non-magical talents freely, using one set to enhance the other. She was not a member of the Thieves Guild, but was fond of Therris in a motherly sort of way. Barenziah warmed to her as she never had toward any woman, and over the next few weeks she told Katisha all about herself.
Barenziah talked to Katisha about her problems with men. Or what she thought of as her problems with men. Katisha shook her head and told her she was looking for love, not sex, that she'd know the right man when she found him, that neither Straw nor Therris was the right one for her.
After Therris's betrayal, Barenziah left Riften and never saw Katisha ever again.
She is mentioned in The Real Barenziah, Book II.