Several parts combine into one character.

相 is a compound ideograph of 木 (tree) and 目 (eye) — examining a tree (木) closely with the eye (目). From "to look closely" the meaning branched into "to face each other → mutual," "the aspect or physiognomy judged by looking," and the "chief minister (宰相)" who observes and assists the ruler.

Korean hanja-sound "sang": 相互 (sangho, mutual), 相對 (sangdae, relative / counterpart), 首相 (susang, prime minister), 觀相 (gwansang, physiognomy), 眞相 (jinsang, the truth), 樣相 (yangsang, aspect). All three branches — "mutual," "minister," and "aspect" — live on.

Mandarin: xiāng, 1st tone (mutual) / xiàng, 4th tone (aspect, minister, photograph). 互相 (hùxiāng, mutually), 相信 (xiāngxìn, to believe), 相同 (xiāngtóng, the same) / 照相 (zhàoxiàng, to take a photo), 相片 (xiàngpiàn, photograph), 首相 (shǒuxiàng, prime minister). A one-character, two-tone case — "mutual" is xiāng (1st), "aspect / photo / minister" is xiàng (4th).

Japanese: on-readings ソウ (sō) and ショウ (shō), kun あい (ai). 相手 (aite, partner / opponent), 相談 (sōdan, consultation), 首相 (shushō, prime minister — a ショウ reading), 相撲 (sumō, sumo), 真相 (shinsō, the truth). The 相 of the Japanese 首相 (prime minister) is this.

Memory aid: examining a tree (木) with the eye (目) — looking at each other, the aspect judged by looking.