How do you make a mnemonic for the word this? You don’t. But, we can try. You climb many hills. On top of one of the hills is a little drop of water. You pick it up and say “THIS… THIS IS WHAT I WAS LOOKING FOR!”
This particular drop of water hold the core (これ) of the universe. If you didn't find this very droplet, we all would have died and this universe would be gone.
Currently there are no similar looking kanji which exist in the WaniKani learning system