sekimen 赤面

In Japanese, sekimen 赤面, meaning "red face," refers to someone's face as it's red with embarrassment or something like that. It's how you say "blushing" in Japanese.

Character: Suou Tamaki 須王環
Anime: Ouran High School Host Club, Ouran Koukou Hosuto-Bu 桜蘭高校ホスト部 (Episode 1)

• Rendering
• Vocabulary

Rendering

In colorful anime the characters' faces do become red as they blush, but in black and white manga that's not possible, so diagonal lines are used instead to represent blushing.

In pure text, three slashes are used to represent blushing. This sometimes happen in text balloons when the character isn't visible in the panel.

• minaide///
  見ないで／／／
  Don't look [at me]. *blushing*
  • surasshu
    スラッシュ
    A slash (/).

Vocabulary

The word aka 赤 means the color "red," while seki せき is the on'yomi of the kanji. The men 面 morpheme can mean either the "surface" or "front side" of something or someone's "face."

Character: Kamado Tanjirou 竈門炭治郎
Anime: Kimetsu no Yaiba 鬼滅の刃 (Episode 23)

• makka
  真っ赤
  "True red." A very strong red color.

Manga: Karakai Jouzu no Takagi-san からかい上手の高木さん (Chapter 2, プール)

• Context: Takagi 高木 sees the outcome of her teasing.
• a-ha-ha-ha-ha.
  あはははは。
  *laughs.*
• kao, makka-kka ni
  natteru yo!
  顔、真っ赤っかになってるよ！
  [Your] face, [it's] turning really red!
• a-ha-ha-ha.
  あははは。
  *more laughs.*

Character: Keith Claes キース・クラエス
Anime: Otome Game no Hametsu Flag Shika Nai Akuyaku Reijou ni Tensei Shiteshimatta…, 乙女ゲームの破滅フラグしかない悪役令嬢に転生してしまった… (Episode 4)

There are various ways to say "to blush" in Japanese. For example:

• sekimen suru
  赤面する
  To "red-face." (as a suru-verb.)
  To become red-faced. To blush.
• kouchou suru
  紅潮する
  To blush. To redden.
  • kao wo kouchou saseru
    紅潮させる
    To redden [one's] face.
    To blush.
• kao ga akaku naru
  顔が赤くなる
  [Their] face becomes red.
  [They] blush.

• hoo ga akaku somaru
  頬が赤く染まる
  [Their] cheeks dye red. [Their] cheeks become red.
  [They] blush.
• hazukashisa de hoo wo akaku someru
  恥ずかしさで頬を赤く染める
  To dye [one's] cheeks red with embarrassment.
  To blush with embarrassment.
  • somaru, someru - ergative verb pair.

For completeness, the makeup term for "blush" would be:

• hoobeni
  頬紅
  Rouge. Blush. Cosmetic product used for coloring the cheeks.
  Literally "cheek-red" in Japanese.
  • The word "rouge" means "red" in French.
  • kuchibeni
    口紅
    Lipstick.
    Literally "mouth-red."

There are various reasons for someone's face to turn red. Embarrassment, shame, bashfulness are pretty common ones.

Character: Fujiwara Chika 藤原千花
Anime: Kaguya-sama wa Kokurasetai ~Tensai-Tachi no Ren'ai Zunousen かぐや様は告らせたい～天才たちの恋愛頭脳戦～ (Episode 9)

• hazukashii
  恥ずかしい
  Embarrassing.
  • hazui
    恥ずい
    (slang, abbreviation.)
  • shinu hodo hazuhashii
    死ぬほど恥ずかしい
    Embarrassing enough to die.
  • hazukashikute shineru
    恥ずかしくて死ねる
    So embarrassing that [I] could die.
• haji
  恥
  Shame. Dishonor.
  • haji-shirazu
    恥知らず
    Not-knowing shame. Shameless.
  • shuuchishin
    羞恥心
    Shame. Shyness.
• tereru
  照れる
  To be shy. To be bashful.
  • tereya-san
    照れ屋さん
    Someone who is shy.

Another common reason would be from falling love.

• dokidoki suru
  ドキドキする
  (For one's heart) to beat.
  To feel one's heart beat faster.
• mune ga kyun to suru
  胸がキュンとする
  (To feel one's) chest tighten.
  For one's heart to skip a beat. (when they fall in love, for example.)

Manga: Karakai Jouzu no Takagi-san からかい上手の高木さん (Chapter 1, 消しゴム)

• doki'
  ドキッ
  *thump*

Other strong emotions can also cause blushing, like excitement.

• wakuwaku
  ワクワク
  *excited*
• koufun
  興奮
  Excitement.
• furueru
  震える
  To tremble.

Manga: Gabriel DropOut, ガヴリールドロップアウト (Chapter 3)

• Context: Kurumizawa Satanichia McDowell 胡桃沢=サタニキア=マクドウェル feels goosebumps from imagining herself doing evil things.
• zoku'
  ゾクッ
  *tremble*

Lastly, it could be the character is just sick.

• netsu
  熱
  Warmth.
  Fever.
• kaze
  風
  Wind.
• kaze
  風邪
  Cold. Influenza.
  • samui
    寒い
    Cold. (weather, temperature.)
  • tsumetai
    冷たい
    Cold. (to the touch.)
    Heartless.
• kaze wo hiku
  風邪を引く
  [To catch] a cold. (literally "to pull a cold.")