Seiko Ayase (綾あや 瀬せ 星せい 子こ , Ayase Seiko?) is a major supporting character of the manga series Dandadan.

She is a well-known spirit medium who also goes by the name Santa Dodoria (ドドリア三太, Dodoria Santa?) and the grandmother of Momo Ayase. While performing her duties, Seiko provides support to Momo and her friends as they become involved with spirits, aliens, and other paranormal activities.

Despite being a grandmother, Seiko has the appearance of a slender woman in her prime. Seiko possesses long snow-white hair that she generally ties up in a large top knot, wears red rectangular glasses. She is a chain smoker and can almost always be seen smoking cigarettes. On top of her youthful appearance, Seiko has been noted for her beauty that she claims others might go so far as to describe her as being bewitching. Due to this, many have mistaken her to be Momo's mother and leave them in shock once they learn her true relation to the girl. In terms of height, she is shown to stand no taller than her granddaughter.

When out in public, Seiko wears a yellow open large button cardigan that reveals her cross-strap top, a blue baseball jacket that has white sleeves with a symbol on the back, hoop earrings, and sandals. When at home, Seiko is shown to wear a white tank top, shorts, a beige haramaki, and light grey arm warmers with matching thigh-high stirrups.

Seiko has a phlegmatic personality and a highly uncouth manner of speech. At the same time, she is selfless and empathetic to help others in need if it's within her power, with the greatest example being her taking people into her house if they need a place to stay. As such, she follows the concept of "being in it together". Instead of accepting Okarun's generous compensation for looking after him while he was cursed by the Turbo Granny, Seiko told him that the only way he could repay her was to help other people who were in trouble.