Ninja
noun
A concealed someone is someone who can conceal themselves. Who's better at that than anyone? A ninja.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Watch out for the rendaku.
コウイチは忍者に、正々堂々と勝負で勝った。
Koichi beat the ninja fair and square!