KanjiJLPT N2
Complete

complete, finish

Mnemonic

Standing on a diving board, you take one last leap into the ocean. At the bottom, a giant fish hook catches your ankle—you've reached the very end. A voice booms: "Journey complete!" The hook won't release until you accept it's truly over.
Additional thoughts
Picture the kanji's shape: the diving board is the top horizontal stroke dropping down, and the fish hook is the curved stroke at the bottom. The finality of being hooked at the end of your dive reinforces the sense of total completion—nothing left to do.
Quick recall
Leap off the diving board, get snagged by a fish hook at the bottom—dive complete.

Details

The keyword for 了 is complete. This kanji conveys the sense of finishing, concluding, or bringing something to an end. It carries the idea that something has been fully accomplished or understood, with nothing remaining to be done. In its essence, it represents the state of completion or the act of wrapping things up entirely.
On'yomi
りょう