Final answer:
Lassiter believes that Isabel would be a good spy.
Step-by-step explanation:
In the given text, the character who believes that Isabel would be a good spy is Lassiter. Lassiter suggests to Jane Withersteen that she should act natural and pretend to go over to the table and then quickly make a move for the door to test if her women are spying on her. This implies that Lassiter trusts Isabel and believes she has the skills to be a good spy.