For context, the sentence is にへら、という気の抜けた笑顔。 And the sentence before is そこまで考えて、胸内にふと一人の人物の顔が浮かんだ。 The character whose monologue this is is spacing out during a movie thinking about his recent past, if it helps.
Could someone help me understand what にへら means? I looked it up and only found a Japanese question about にへら but it was in the middle of a sentence, so I didn’t think it applied.