Oblivious to Love: She completely misreads Kei's intentions, leading a jealous Yuu to fight with her. Not to mention how she fails to even register Michael's Stalker with a Crush feelings towards her. Also, despite being in love with him, she was even oblivious to Yuu's feelings towards her until he spelled it out for her.
Passionate Sports Girl: To a small degree. She's very athletic and a member of the school's tennis team, not to mention she's a rather decent player in her own.
She Cleans Up Nicely: Although initially regarded as plain and not beautiful enough, true to shoujo manga tropes, Miki is still evidently very pretty, judging by her popularity that was enough to make Ginta afraid of telling other guys that he liked her, not to mention that the mangaka Yoshizumi Wataru based her looks off a magazine it girl.
Noted here and there in the manga, like when Yuu says she looks cute in the dress she buys from his workplace. Is noted by Arimi as well when she sees Miki with her hair down for once, and definitely by Kei (and Michael, in the anime) who fall head over heels for her at first sight.
Over a decade later, Miki is drawn as pretty and cute even into her thirties, inside of the 2013 sequel series, Marmalade boy Little.
In the scene near the end where Yuu yells at her asking her not to love him anymore, she changes the subject and goes off happily to look at a garden of sunflowers by someone's house. Yuu quickly sees that she was trying not to let him see she was about to cry.
Tomboy and Girly Girl: She could be seen as the tomboy to Meiko, Arimi and Anju's girly girls, although she does not feel one way or another about the subject.
Boy Next Door: Literally, and of the stepbrother variant, To Miki.
Chaste Hero: He refused to date with any girl who ever confessed to him. Arimi was the exception as she convinced him to give her a chance, but he still rejected her after a few months containing unbelievably chaste dates. He proudly admits to Miki that his first kiss was with none other than her.
Chick Magnet: The Anime plays this Up to Eleven in the most annoying ways possible. In the manga, he certainly is one, but more tastefully so.
Covert Pervert: With Miki, like when he asks her to sit between his legs on his bed, after she shows off her new uniform with a very short skirt.
Also shown by how eager he is for their trip together. Sure, he appears to be mildly looking forward to it, but for Chaste Hero Yuu who mostly hides his true feelings, that sort of behavior holds much more meaning than it normally would.
Not to mention that upon finding Miki in the infirmary bed a second time, he proceeded to literally climb over her on the bed and tried to kiss her, all while the poor girl is hiding her mouth under the covers.
Good Is Not Soft: Although overly indulgent with Jinny's clingy behavior towards him, possibly due to his own confidence in his Single-Target Sexuality towards Miki, as well as the Anime's struggles with proper characterization (As Yuu is an entirely different character in the manga) he firmly tells her to back off or else when she insults Miki.
Heroic BSOD: Actually cries when he comes under the belief that Miki is his biological sister
It's Not You, It's Me / Shoo the Dog: When he breaks up with Miki, despite still loving her and wanting her badly, because he (mistakenly) thought he found out that they were siblings.
Jerk with a Heart of Gold: A unique kind. Yuu is by all means polite and sweet, but like Miki says he actually has some quite bitter points. He especially liked to bump into Miki and trip her up, before he admitted he loved her and couldn't very well treat his girlfriend like that anymore. Though he still teased her relentlessly. Underneath it all however, he is a good boy who has almost an unhealthy martyr complex.
Tranquil Fury: Doesn't get angry often, but when he does, it's this.
Miki is visibly scared of him when he corners her in the aftermath of her avoiding him, Yuu being visibly upset and then unsettled at her changing her sweet and lovable attitude in his presence.
To Jinny, when she implies that he should break up with Miki and choose her.
What the Hell, Hero?: Meiko invokes this towards him when he misses too many calls from Miki, and hears that he also supposedly slept with Jinny.
He later invokes this to his and Miki's parents, under the belief that they hid the fact that Yuu and Miki were biological siblings, leaving the step siblings to fall in love too deeply to separate anymore.
Convenient Miscarriage: Was pregnant with Jin's baby before marrying Youji, but it was a stillborn. Her mother wrote a rather confusing letter about it to her... Yuu found it and mistakenly thought it referred to him.
Cool Big Sis: To Miki, since she doesn't want to be pushy to her.
Honest Adviser: To Miki. She's very level-headed, and outright laughs at her when Miki asks her about her supposed date with Yuu. Meiko is usually the one pointing out that Miki's soap opera problems aren't as bad as they could be. She's the voice of reason whenever Miki gets a little caught up in her own drama.
Spoiled Sweet: So much that in the anime, she singlehandedly finances her and Miki's trip to the USA, as a way to repay Miki's support that allowed her to go to Hiroshima and rekindle her relationship with Namura.
Sugar and Ice Personality: She's sugar to Miki and Namura and her friends, but she can flip the ice switch pretty quickly. Even with Miki, she's very reserved and prefers to focus on Miki's issues rather than her own.
Took a Level in Jerkass: While not a bad person in any way, Meiko is much more pushier and snarkier (sometimes being pretty mean) in the anime.
Throw the Dog a Bone: In the anime, he not only is more sympathetic and comes to support Ginta and Arimi, but he gets a girlfriend too.
Lampshaded by the author in the last volume omake: when talking about her characters, she said that she felt sad some of them weren't able to achieve happiness and she accompanied this with a drawing of Satoshi looking dejected and Rokutanda saying "At least I've got a girlfriend in the Anime"
Kick the Dog: Hiding Miki's medallion (or bracelet, in the manga), then giving it to Yuu and saying that she threw it away because she doesn't like him anymore. Yuu isn't fooled, but it still brings a huge rift between them.
Just a Kid: Miki thinks so. He's very unhappy about that.
Genre Savvy: Is very aware of her Unlucky Childhood Friend status and tries to make the best out of it. i.e. she never clings to Yuu and immediately tells Miki that she doesn't intend to break them up.
Ill Girl: Has a serious heart illness that almost kills her.
Death Glare: Gave Jinny a loooong one to take warning, and yet she's stupid enough to despise it.
Doing Research: Whenever he's free for the night, he would usually stay in his dorm attic to study the stars using his telescope.
Gay Guy Seeks Popular Jock: What people around him rumored his relationship with Brian was. But this was only the tip of the iceberg of him being actually a—
Guile Hero: Where his core motive of befriending Brian was to get close and befriend Jinny.
Genre Savvy: Like Anju, while his ultimate goal is to have Jinny become his girl, he's aware of Brian trying very hard to pursue her, while Jinny's trying to court Yuu, and that he himself might be a Hopeless Suitor to her. But he just opts to lay back and continue to observe the situation in hopes to have a chance, while at the same time actively dates Anju to have a Birds of a Feather friend to talk with.
The Smart Guy: It was implied that Jinny would constantly ask him for school notes, and he already has been prepping himself to major in astronomy for college.
Spell My Name with an S: Although the English dub version went with "Bill" note Japanese romanization would spell "Bi-ru" for William's short name, but his official preference in the original Japanese version was actually "Will" note Wi-ru, which is completely legit amongst western culture.
This could be mostly blamed on the Anime's wonky character design.
Kick the Dog: Telling Miki that she was Yuu's "other woman", out of jealousy. This makes the already distressed Miki break up with Yuu in the airport, as she's too emotionally exhausted to handle a Long Distance Relationship.