Aiko Senoo (妹尾あいこ, Senō Aiko) is the third Ojamajo to appear in the seires. Her parents are divorced for most of the series and she stays with her father who is a busy taxi driver.
Aiko serves as the series tomboy. She is mainly a girl who loves adventure and playing around; she does like to dwell on negative thoughts and is often the one to cheer up the group in times of depression. This does not mean that she does not have emotional moments. When dealing with something she loves or is passionate about, Aiko can become very serious and determined. She also can appear more girly and emotional when it comes to her mother.
Along with being a tomboy, Aiko is short-tempered in many situations which can be helpful at sometimes but troublesome at other. She is also one athletic people in her class, despite being a girl. But although she is good in any other sport, she has trouble swimming. Most notably, she has a strong Osaka accent since she only moved to Tokyo recently.
Aiko has blue eyes and short blue hair she wears in split-curls. However, there are times when she her hair in two ponytails tied by bows, as shown in Ojamajo Doremi Sharp episode 20. She is most seen with a light green shirt and tan shoes. In seasons 1 and 2, she wears blue overalls. In seasons 3 and 4, this is replaced by blue pants and a tan vest. When transformed into her Witch Apprentice outfit, she wears blue.
- Nobuko Yokokawa: Aiko was at first annoyed when Nobuko lied about knowing a famous star and after lying about not having a mother. Aiko soon found out she did this to have more in common with Aiko whom she admired and wanted as a friend. Aiko sees Nobuko as a good friend throughout the whole series and looks forward to reading the new stories she makes, containing a main character based off of Aiko.
- Leon: Aiko's FLAT 4 counterpart. He often challenges her to random sports battle (example in Ojamajo Doremi Sharp episode 36) in which she always wins. Leon clearly annoys Aiko though he seems to hold an interest in her; she also points out how she hates that way his uses random english words in his sentences.
- Keiichi Harima: An old friend that appears in Ojamajo Doremi Naisho. She made a promise to marry him when younger to avoid others from finding out she could not swim. He appears later and reminds Aiko of the promise, which she thought he would forget. Aiko does not feel the way about Harima the ways he feels for her but it is clear she does care some about him, as she doesn't drive him away instantly like he does with other guys. The relationship is only explored in one episode so it is unsure what happened between them.
- Aiko's magical crystal is shaped like a diamond.