Denshi Jisho — Online Japanese dictionary

怪 かい mystery; wonder
Noun 
物怪 もけ unexpected
Out-dated or obsolete kana, No-adjective, Na-adjective, Noun, Usually written using kana alone 
奇怪 きかい strange; wonderful; weird; outrageous; mysterious
Common word, Na-adjective, Noun 
勿怪 もっけ unexpected
No-adjective, Na-adjective, Noun, Usually written using kana alone 
物怪 もっけ unexpected
No-adjective, Na-adjective, Noun, Usually written using kana alone 
妖怪 ようかい ghost; apparition; phantom; spectre; specter; demon; monster; goblin
Common word, Noun 
奇怪 きっかい strange; wonderful; weird; outrageous; mysterious
Na-adjective, Noun 
幻怪 げんかい strange or mysterious and troubling
Na-adjective, Noun 
幽怪 ゆうかい ghost; apparition; spirit
Noun, Archaism 
奇っ怪 きっかい strange; wonderful; weird; outrageous; mysterious
Na-adjective, Noun 
物の怪 もののけ (vengeful) ghost; specter; spectre
Noun 
醜怪 しゅうかい monstrous; hideous; ugly; bizarre
Na-adjective 
奇奇怪怪 ききかいかい very strange; bizarre; weird; mysterious
Na-adjective, Noun 
狐狸妖怪 こりようかい foxes and badgers (which used to be believed to bewitch humans) and all sorts of bogies; a sly fellow who does evil by stealth
Noun 
複雑奇怪 ふくざつきかい complex and mysterious (bizarre); complicated and inscrutable
Noun, Na-adjective 
怪中の怪 かいちゅうのかい mystery of mysteries
Noun 

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