Thorn
し, じ
いばら
None
If you look at flowers the first thing you notice are the leaves and petals themselves, then the next thing you notice are the briars. You never see thorns first, only after you've looked at the flowers.
When you see the briars you can't help but yell, Eee! Ba riars! (いばら)! You pause funny because you prick your finger on one while you're yelling. Ouch!
Currently there are no similar looking kanji which exist in the WaniKani learning system