Lightning Background Effect

In manga and anime, a lightning is used as background effect in panel behind a character, meaning that they're shocked, as in, startled, dumbfounded, astonished.

In Japanese, this is called inazuma furasshu イナズマフラッシュ, "lightning flash," or kaminari furasshu カミナリフラッシュ, "thunder flash." The word "flash" refers to the sparks along the lightning, which are instances of Beta Flash.^(tonbi.jp)

Akino Sakura 秋野桜, example of osoroshii ko 恐ろしい子.

Anime: Ore wo Suki nano wa Omae dake kayo 俺を好きなのはお前だけかよ (Episode 2)

Examples

For reference, some examples:

Manga: Gabriel DropOut, ガヴリールドロップアウト (Chapter 9)

Context: Raphiel ラフィエル plays Satania サターニャ like a fiddle.
douka onegai shimasu
Sataanya-san...
どうかお願いしますサターニャさん･･･
Please [do it for me], Satania-san.
ie
Sataanya-sama
いえ　サターニャ様
No. Satania-sama.
zugyaan
ズギャーン
*kaboom*
Here, Raphiel makes Satania feel important by using the ~sama ～様 suffix, which has added reverence. Raphiel only used the suffix here to compel Satania into doing something for her, but nevertheless managed to fawn her over with this display of blatant flattery.

Marie マリー doing a rumic hand sign with a lightning background.

Character: Marie マリー
Anime: Tatoeba Last Dungeon Mae no Mura no Shounen ga Joban no Machi de Kurasu You na Monogatari, たとえばラストダンジョン前の村の少年が序盤の街で暮らすような物語 (Episode 3)