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 decide to remain behind. Everyone else leaves, and you think you're alone. Then, someone knocks (のこ) on the door. Your heart jumps in your throat. They knock again.
Please stay behind if you think you're mysterious.
We left for Portland, but Koichi remained behind in Japan.
This will go down in history as the best Japanese learning material ever.