暖まる
あたたまる · atatamaru · JLPT N3 · verb-godan
to warm up
☀️ 暖 = sun 日 + 爰 · sun-warmth; same root as 暖かい (atatakai, warm)
The kanji 暖 has the sun radical 日 on its left. The -まる ending means something warms up by itself, like a room or your body. Its partner 暖める (atatameru) means you warm something up.
暖 — sun radical 日 + 爰 (yuán, phonetic)
暖 is a semantic-phonetic compound. The 日 'sun' radical carries the meaning of warmth, while 爰 supplies the sound (historically close to 'nuǎn'). It originally described the warmth of sunlight and broadened to mean 'warm' in general — both physical heat and figurative warmth like kindness.