KanjiJLPT N5
Same

same, agree, equal

Mnemonic

You enter a tunnel and discover one enormous mouth carved into the stone wall. Whatever you say, it repeats back perfectly — everything sounds exactly the same. That single mouth in the tunnel echoes the same words forever, proving nothing ever changes down here.
Additional thoughts
Picture the tunnel shape (冂) surrounding the one horizontal stroke (一) with the mouth (口) sitting below it. The echo chamber visual reinforces sameness — identical sounds bouncing back unchanged.
Quick recall
One mouth in a tunnel echoes everything back the same.

Details

The keyword for 同 is same. This kanji conveys the idea of sameness, agreement, or being identical. It is used to express that two or more things share the same quality, status, or identity, and it extends to meanings of togetherness and unification, as in people coming together under a common purpose or shared condition.
On'yomi
どう
Kun'yomi
おな.じ