Witness
noun
An evidence person is a witness, because witnesses are people who provide evidence (you know, with their mouths or something).
しょうにん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
証人が、ビエトが有ざいであることを証明してくれました。
The witness proved that Viet was guilty.
私の夫のさいばんでは、夫の口座がある銀行の行員の人たちが証人として出ていしてくれました。
In my husband's court case, the clerks of the bank where he has his accounts appeared in court as witnesses for him.
証人として、遠回しに言わずに率直に君の質問に答えさせてもらうよ。ああ、あのフグが確かに君のプリンを食べていたね。
As a witness, I won’t beat around the bush, I’ll answer your question frankly. Yeah, that Fugu was absolutely eating your pudding.