Hewlett Aldershot III was the second-in-command of the United States Secret Service.
Because of his status, the Yeerks made Aldershot a target for infestation, ordering their operative Chapman, to engineer a car accident so that he would be sent to hospital and thus easier to get close to. The Yeerks then infiltrated the hospital staff, intending to infest him while he slept.
The Animorphs were informed of this plot by the Chee and moved to prevent the Yeerks from infesting Aldershot, by utilizing mosquito morphs and acquiring his DNA. But they were interrupted when a freak morphing anomaly which pulled them into Z-space. When they returned, they found that Aldershot had awakened of his own accord. He was presumably infested after awakening from his coma.