In the past and at younger age, Miwa used to be naive, childish and nervious. Even if she was really gentle and empatic, she usually got scared when she was scolded, and acted nervious in difficult situations. Still, she always tried to make peace around her and that everyone treated each other correctly. She always forced herself to follow the rules and be fair with everyone, treating everyone as equals. Even if she was a bit shy at the time, she encouraged herself to do right and be independant from others, resolving her problems alone.

Growing up and because of experience, she changed some of her behaviors around others. As always, she was the mediator between people but she acted calmly and didn't looked terrified at the circumstances. She learned to keep calm even in the worst situations, and act acording to it. Obtaining knowledge from others made her make quick answers to mostly everything. She also developed a motherly role around her circle, trying to be the figure that keeps everything in order and that others can rely on. She has that aura around her.

Miwa is someone who tries her best to protect her loved ones and make their lives the best. She has a gentle soul and is able to help others even if she doesn't get any help with her own problems. She has a lot of focus for task and is able to finish them quickly since she is always busy with something because she is someone who is mostly unable to say no to someone.

She is a traditional person. Even if she has been a shinigami for a long time, she doesn't go to the human world that much, so isn't used to the current culture. Because of her lack of knowledge on it she is afraid and nervious around humans, and prefer to stay inside. She is too ashamed to even wear normal human clothing and always go for the traditional oufit, wearing kimonos, yukatas, hakamas, etc, since those are more comfortable for her. She is unable to wear a swimsuit, saying that is too revealing, but she acts completely normal in an onsen in the soul society.

Even if it may seem that she is mostly serious and calm, is very easy to make her laugh, even laughing at bad jokes. Miwa tends to be so emotional about romantic things that she usually cries watching movies or reading books, her favourite genres being romance and mystery. She is also really interested in human festivities, that she celebrates with all her heart, being christmas her favourite one. Even if she likes romance, Miwa is unable to understand Valentine's day and White day, since she thinks that everyone should show your emotions any day of the year. She don't hesitate to show her feelings to others, since mostly of them are caring and love.

Miwa was born as the first child in the Seika family, getting inmediatly the position of the 20th head of the Seika clan. She had to inherit that role at young age because of the death of her father at Hollows hands when she still was a kid. Not much time after her father died, she joined the Shino Academy to become a shinigami so she could follow the tradition of her clan, where the head of the family took a shinigami high seat position to keep a high status.

During her academy years, Miwa got a reputation, being hated by some but also liked by others because of her point of view in the Soul Society hierarchy. Miwa always treated everyone respectfully, no matter where they came from, and tried everyone to respect each other. It was mostly nobles that didn't liked her trying to lecturing them around, and used to confront her, but Miwa always got away thanks to her Kido abilities.

During her first year, Miwa was Urahara Kisuke's classmate, and thanks to an incident where Urahara stood up for Miwa from other students, they slowly to became friends.
She graduated from the academy 2 years earlier and got inmediatly in the Gotei 13, having a seated place in the 12th Division in charge of Hikifune Kirio. Miwa got Hikifune's attention thanks to her inmense spiritual power. Since Hikifune herself was making studies in how improve the Shinigami's labor, she thought that Miwa with her power could be a good companion for investigation and work.

After Hikifune's promotion to the Royal Guard, Miwa and Sarugaki Hiyori took care of the 12th Division with the lieutenant as the voice of authority while Miwa worked on the office, taking care of the paper work and assignments.
When Urahara took the place of the Captain of the 12th Division, Miwa's duties didn't changed that much but also had to be a kind of dealer or peace maker between Urahara and Hiyori. Since she knew both of them beforehand, Miwa was able to convince Hiyori to be a bit more understanding, but also to help Urahara with his lack of confidence in being a captain.

Alongside with other members, she formed part of the S.R.D.I. as a researcher even if her main role was more of a supervisor. She used keep peace in the facility and helped everyone to keep them working, doing things like bringing them some snacks or ask if they needed something extra.

When the incident with the dissapearing souls began, Miwa started helping Urahara creating the artificial bodies, and almost went with Hiyori to take some samples but was stopped by Kurotsuchi who needed help checking the laboratory inventory. After the emergency state began, she couldn't help but worry about Hiyori and because of Urahara's behavior with the issure, she thought he was going to do something about it. Being right, she saw Urahara sneaking out the barracks and asked him to come back safely with Hiyori, seeing him go with Tsukabishi Tessai's company.

She couldn't sleep because of her concern, and went early to the barracks after barely sensing something around it. That's when she witnessed Urahara and Tessai took under custody, and barely saw the masks on Hiyori and the others.
Miwa learned later that they all disappeared alongside with Shihouin Yoruichi, starting to belive that Urahara was the one behind the experiments and was responsable for all the atrocities others where talking about. After the incident, she took care of the 12th Division alongside with Kurotsuchi Mayuri, but because of their morality differences they used to argue most of the time, and in the end, Miwa was promoted to the 5th Division when Kurotsuchi became captain in the 12th Division.

Years later, Miwa went to Karakura Town in a mission, being surprised by modified hollows, and was injured in the battle. She was helped by Urahara, who sensed her spiritual pressure, and took her in while Miwa recovered. Still, Miwa distrusted Urahara due the past, and was determinated to ask for reinforcements in order to arrest him and Tessai. Even so, she listened to Urahara's side of the story and calmed down.
Miwa wasn't sure in what to belive at the time, but she changed her opinion one she was told that the ones that where partially transformed into hollows where actually alive, meeting them, getting a relieved and happy feeling to see Hiyori again.

After that, she returned to the Soul Society, but started being more careful about her captain, Aizen, and the other culprits.

During the time of the ryoka invasion Miwa kept a low profile with the incidents.
She started having doubts about it after knowing where the humans came from, knowing that it was Urahara's doing and there was a meaning behind it. After captain Aizen's fake death and the imprisonment of the lieutenant Hinamori, Miwa got in charge of the 5th Division alongside captain Hitsugaya, having a lot of work with paperwork that the humans in soul society weren't too important for her at the moment.

Still, under the confusion, she accompanied captain Unohana and lieutenant Isane to the Central 46, having a bad feeling about Aizen's death. When she knew the truth revealed by Aizen, she fully trusted Urahara again, helped the ryoka and took charge of the 5th Division when everything calmed down again.