Rumi mentioned she would bully other girls with her classmates, but only because everyone else did it. They would get bored after a while and pick a new girl to bully. However at some point they started picking on Rumi and haven't moved on. She is not known to have had any friends before this and Yukino believes she has always been trying to fit in.
In the Chiba village, Rumi & her peers were spotted by Yukino during the treasure hunt event for the elementary school students. Yukino noticed Rumi was openly outcast by the four girls, who are Yokko, Mori, Hitomi, and Yuka. Rumi was mostly seen following the group of girls and is always a few steps behind them. Every once in a while, the other girls would be seen looking over their shoulders and stifling soft snickers among each other probably a gossip conversation about Rumi.
Hayato at first tried to resolve the situation by trying to include her in the group. Instead of helping, this attracted more hateful and jealous attention towards her. She finds her way to Hachiman and Yukino, whom she realizes are different than the other high schoolers and the same as her. But she refused to accept Yui as one of them and believes Yui belongs to the social side (Hayato's gang). They talk about making friends but Rumi dismisses it by saying she'll just wait until middle school to make friends since people from other areas would be there. But Yukino explains that most kids in her school would go to the same middle school. Rumi's past would be treated as a joke and negativity could even follow her to high school.
Rumi expresses the misery that she is going through, her friends turned into bullies. Her secrets became a conversational topic and joke for others, she continues by saying that a person who talked to her a lot was bullied before and lament that she didn't do any thing wrong to deserve this. According to Hachiman, they didn't need a reason, they just want to have fun at the expense of others.
The next day she finds Hachiman at the river bank and they are joined by Yukino and Yui. She states that everyone left her behind after finishing breakfast. She asks Hachiman, if he had friends back when he was in grade school, to which he replies negatively and says that goes for most people. Even so, Rumi clutches her camera and says that it's not an acceptable explanation for her mum, who bought the camera for Rumi so that she can take pictures with her friends. Rumi says that she doesn't want to be friends with them anymore, but feels bad being left-out. Hachiman hints his plan to help her before leaving.
Hachiman has a plan to destroy the bonds between Rumi and her peers. He has Hayato's clique act as bullies threatening Rumi's group during test of courage event and exposing their selfish nature to each other. Hachiman believes this will cause them to turn on each-other. This should prevent further organized acts of teasing Rumi. Hayato bets that Rumi's group will deal with it together.
Komachi lets Rumi's group into the forest last. They were lead in a completely different direction than the other groups. Hayato's group confronts Rumi's group and tells them to leave any three people behind and the remaining will be freed. Rumi was selected as a default sacrifice, then the rest started quarreling about the remaining two members.
When the other members of Rumi's group begin to bicker and argue among themselves, Rumi saves them by using her digital camera's flash to blind Hayato's group and escaped with her peers. Hachiman was impressed by her action and hoped for her better future. Later at the campfire, the girls of Rumi's group still haven't recovered from the shock, while Rumi keeps peeking in Hachiman's direction. Rumi avers her eyes and doesn't thank Hachiman.
The aftermath and effect on Rumi and her peers was never revealed, but it was hinted that due to Hachiman's actions the bullying had stopped. However, Rumi is still a loner.
Christmas collaboration event
Rumi makes an appearance again in Season 2 Episode 7 where it is revealed that she is one of the representatives of her elementary school. They are also participating in the Christmas collaboration event along with Sobu & Kaihin Sougou High School.
Initially Rumi and her peers are unsure of what to do. Rumi is the one to ask Hachiman what they should be doing. He asks them to decorate the tree, since nothing was set on stone. Rumi appears to be the head of her group and relays instructions to everyone else.
However, she is seen making Christmas decorations alone. Hachiman offers to help her, and eventually she accepts his help.
Later after both schools divided their responsibilities, Hachiman asked Rumi to be the star of the Christmas play. Thus making her more popular. It was mentioned in Volume 9 that Rumi was praised by all the audience and peers for her performance including Hayama's clique. Rumi was mentioned to smile throughout the event after it's success and she was no longer lonely.