Izumi is of average height and has a slim figure with pale skin. He is noted to be relatively handsome and attractive with long lashes and a slightly feminine-looking face, a feature that others often point out. He also has deep blue eyes, messy black hair that goes down to his nape with parted bangs, and thin eyebrows. Izumi has a total of nine piercings: four down each ear and one on his lip, all of which were done by himself. He also has a few dark-colored tattoos on his left shoulder, upper left arm, and side.
In his first appearance, Izumi is seen dressing heavily in order to hide his tattoos, wearing his winter school uniform and blazer. He keeps his hair shoulder-length and straight to avoid drawing attention to his piercings and wears thick-rimmed glasses.
Out of school, he dons a more casual appearance and is seemingly indifferent about the fact that others can see his piercings and tattoos. He removes his glasses and keeps his hair tied up into a small ponytail with a few bangs framing his face. Occasionally, he wears a lip ring and chain, along with casual outfits and various accessories.
When he first starts dating Kyouko, Izumi decides to cut his hair short and withdraws from wearing glasses. With his side bangs reaching just past his ears, Izumi's piercing holes become visible, but a few piercings—specifically the one on his lip—close up as a result of him not using them. His new appearance sparks the attention of many, and he is considered to be quite attractive by the girls in his class, much to Kyouko's annoyance. He is seen wearing his blazer fully buttoned up less often, opting to wear the buttoned school sweater over his uniform instead.
Initially at school, Izumi gives off the impression of a quiet, gloomy, anti-social person. His classmates' perception of him is that he is an otaku with good studying capabilities, but a hard person to approach and maintain a conversation with.
Izumi was noticeably shy and struggled to interact with others when he was a child up to his time junior high, resulting in him being frequently ostracized and bullied—especially by Makio Tanihara and his friend group—as his peers considered him a dreary and distant individual. This treatment caused Izumi to adopt a pessimistic nature, and he would give himself piercings with a safety pin whenever he felt school was giving him a difficult time. During this time, it was implied that he also contemplated suicide.
However, one day, Izumi met Kouichi Shindou, who was, at the time, friends with Makio. Kouichi treated Izumi much better than their other classmates and the two became good friends, even with Makio's attempt at interference, and they kept in touch despite going to separate high schools. The two are close enough that Izumi changes his personality into a violent and foul-mouthed temper towards Kouichi, a complete 180 compared to his usual demeanor. Izumi credits Kouichi for his positive change in personality between junior high and high school.
Between the beginning of high school to his third year, Izumi still retains a dark aura that his classmates consistently point out. However, he himself is less pessimistic (likely due to Kouichi's influence), and a younger Kyouko points out that Izumi isn't as gloomy as others make him out to be after the two pair up for a project.
When Izumi and Kyouko meet each others' hidden sides for the first time, Izumi has a friendly and laid-back attitude, a trait Kyouko unanticipated. When Kyouko doesn't make an effort to avoid him the next day at school and explains her reasoning, Izumi is noticeably happy about her actions. Izumi is also quite laid-back towards his studies and goes along with the belief that everything is fine as long as he is able to graduate, resulting in him doing poorly on tests and assignments. Although he has a tendency to mispronounce certain Japanese words, Izumi is quite good at tongue twisters.
Upon interacting with more of his classmates, Izumi is shown to be a good friend and a caring person that often goes out of his way to help others (especially towards Kyouko and Souta, after realizing the hard work Kyouko put in to maintaining her home and taking care of her brother). He has violent tendencies when his friends are involved in argumentative affairs and may resort to physical attacks if he deems them necessary.
At times, Izumi can be rather dense and impulsive, which is likely the reason behind why he got his tattoos that force him to dress in layers and take extreme measures so as to not reveal them to others at school. Although he is careful to hide his secret, there are close calls that result from Izumi’s dense nature. Outside of school, he has the freedom to show off his piercings and tattoos and has no trouble displaying them.
He appears to have very low self-esteem and is very awkward when talking to people he's not close with, often saying he can't connect well with others. He worries that it would be embarrassing for Kyouko to be seen with him. Miyamura hates drawing any sort of unnecessary attention towards himself, preferring to be treated as "invisible" in class. However, because he feels that Hori is being made fun of for dating him because of how he looks, which leads him to cut his hair, and also stopped him from wearing glasses to school. This causes a lot of girls to start approaching him and commenting on how good looking he is, even flirting with him and snapping pictures of him with their phones much to Kyouko's annoyance.
He is very afraid of anything related to horror or gore but usually watches horror movies because Hori enjoys them. It is unknown if he is a sadist or a masochist, however he often hits Shindou and aims sharp remarks towards him, especially regarding the fact that Shindou had to repeat a grade due to poor attendance. He does not seem to hold any sadistic feelings towards his girlfriend except that one time he decided to put his foot on her head because he wanted to bully her back for once since he is the one always getting pushed around. In fact, he has problems treating anyone impolitely or violently towards anyone except Shindou or occasionally Sawada.
- The name Izumi means "that one" (伊 i?) and "clear, pure" (澄 zumi?).
- Izumi's surname Miyamura means "temple, shrine, palace" (宮 miya?) and "town, village" (村 mura?).
- Izumi works in his family's bakery after school.
- Izumi is very good at tongue twisters.
- Izumi can't pronounce the words "atatakai" (meaning warm), "katatataki" (meaning shoulder massage), "yamabuki-iro" (meaning bright yellow), and "zashiki warashi" (a childlike guardian spirit), instead says "akkatai", "kakatataki", "mamabuki-iro", and "dashiki zarashi".
- Izumi's Japanese score is a two out of five (2/5).
- Izumi had 10 piercings in total which two of them closed, one on the lip and one on the tongue.
- In Chapter 13, it is revealed that Izumi is afraid of horror movies.
- Izumi has a full mark in Health, which he usually uses against Tooru Ishikawa.
- Izumi won first place in the Horimiya character popularity contest on November 17th.
- Izumi dislikes tomato peel.
- Izumi likes women who wear blazers.
- On the webcomic ending, Izumi and Kyouko Hori had a son named Kyouhei Miyamura.
- Izumi is an only child.
- Horimiya Manga: Chapter 21 (p. 20-21).
- Horimiya Manga: Chapter 1 (p. 21).
- Horimiya Manga: Chapter 3 (p. 10).
- Horimiya Manga: Chapter 23 (p. 8).
- Horimiya Manga: Chapter 38 (p. 10).
- Horimiya Manga: Chapter 1 (p. 16-17).
- Horimiya Manga: Chapter 26 (p. 22).
- Horimiya Manga: Chapter 10 (p. 10).
- Horimiya Manga: Chapter 8 (p. 4).
- Horimiya Manga: Chapter 15 (p. 10).