He might yet establish himself as the benefactor of the family.
On the way to the Castle he was so mysterious with Robert Black that the stout constable became a prey to mingled curiosity and doubt.

He could not make up his mind whether William Roper really knew something of importance or was merely vapouring.

William Roper neither gratified his curiosity, nor banished his doubt.

He was alive to the advantage of reserving his information for the most important ear, so as to gain the greatest possible credit for it.
At the first sight of him Mr.Flexen felt that he had before him an important witness, for he took a violent dislike to him, and he had observed, in the course of his many years' experience in the detection of crime, that the most important witness in hounding down a criminal was very often of a repulsive type, the nark type.

William Roper was of that type, but his story was indeed startling.
He first told how he had seen Colonel Grey kiss Lady Loudwater in the afternoon--Mr.Flexen noted that Lord Loudwater had accused her of kissing Grey--and of their spending most of the afternoon in the pavilion in the East wood.