Scary
Frightening, Eerie, Dreadful, I'm Afraid, I'm Scared
interjection, い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of fear? Scary or frightening. It can also mean I'm afraid or I'm scared too.
こわい
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You know what's really scary? Koalas (こわ). Have you ever heard one of those things? They're super scary, don't go near them.
俺の母親は世界で一番恐い。
My mother is the scariest in the world.