Several parts combine into one character.

君 is a compound ideograph: 尹 (yǐn, "to govern" — depicting a hand holding a staff of authority) + 口 (mouth). The composite reads as "one who holds the staff and issues commands with the mouth" = ruler, sovereign, lord. From this image of supreme authority grew an opposite extension: the character also became a friendly second-person address — "you / friend" — echoing how English "lord" gives us "milord" but also "lordy" colloquial address. Few characters span the social register from "monarch" to "buddy" in one form.

Korean reading "gun." 君主 (gunju, monarch — formal political term), 君臣 (gunsin, "ruler and vassal" — Confucian relationship pair), 暴君 (pokgun, tyrant), 郞君 (nanggun, "young lord" — archaic affectionate term for a husband, used in classical Korean literature), 夫君 (bugun, "honored husband" — formal way to refer to someone else's husband). Korean keeps 군 in formal and historical registers.

Mandarin jūn, 1st tone. 君 (jūn), 君主 (jūnzhǔ, monarch), 暴君 (bàojūn, tyrant), 君子 (jūnzǐ, "noble person / gentleman") — perhaps the most consequential Confucian moral concept. The Analects defines the 君子 as the morally cultivated person, contrasted with 小人 (xiǎorén, "petty person"). Two and a half millennia of East Asian ethical thought revolves around the question of how to become a junzi. 诸君 (zhūjūn, "all you gentlemen / everyone") survives as a formal address.

Japanese on-reading クン (kun) — 君主 (kunshu, monarch), 暴君 (bōkun, tyrant), 君臨 (kunrin, to reign / dominate). Kun-reading きみ (kimi) — 君 (kimi, "you / lord"). Critical Japanese cultural compound: 君が代 (Kimigayo, "Your Lord's Reign") is the title of Japan's national anthem, sung at every ceremonial occasion since the Meiji period. The lyrics are a 10th-century waka poem wishing the emperor's reign to last "until pebbles grow into boulders covered in moss." Also in informal modern Japanese: 〜君 (kun) is a familiar suffix used by superiors addressing male juniors or by close peers (Yamada-kun "Yamada-bro") — much more familiar than the formal 〜さん.

Memory aid: hand on staff (尹) plus mouth (口) — the figure who commands with authority = ruler.