Jake is the leader of the Animorphs, as well as the first person to narrate a book in the series.
A close friend of Marco at the beginning of the series, Jake at first seemed to be the epitome of normality. Of average height for his age, and with brown hair and eyes, Jake didn't look particularly noteworthy, either.
At first, Jake had the same reluctance to oppose the Yeerks as Marco, but he soon found out that his older brother Tom was a Controller: someone who had been infested by one of the Yeerk parasites, and was forced to serve their will. This gave him the resolve he needed to join the fight, though he remained somewhat reluctant to lead. When the Animorphs next encountered Visser Three, after their second attempt at infiltrating Chapman's house had ended in Rachel's capture, Jake used his Tiger morph, his first and primary combat morph, to deal with the Hork-Bajir and Taxxons aboard the Blade Ship.
When the Tobias discovered the presence of a Yeerk supply ship - known to them as a truck ship - Jake was willing to go along with Tobias' suggestion that they investigate it. He, along with the other morph-capable members of the group - which did not include Tobias at that point - was trapped in the hold of the supply ship with the water rising all around them. With seemingly no hope, Jake and the other Animorphs demorphed; if they were going to die, they would die fighting.
Jake was the first one to find out about the Radam's covert invasion of Earth, thanks to the information he received from the Tekkaman named Starfire, who had been almost fatally injured in a space battle with a force of Teknomen long prior to the events in the main series.
Leading the other Animorphs into the forest, following Starfire's guidance - though the Tekkaman was unknown to him at the time, and he thought it was the Ellimist - he quickly came to the Radam's hidden base. It was buried beneath the forest floor, all the better to feed on the rich nutrients of the soil, so no one coming upon it would be able to discover it until it was too late. Demorphing, having been informed that it was the only way to access the base, Jake fell inside.
While he remained in his human form, Jake was vulnerable to detection by the Radam's life-form sensors, and hence to being dragged into one of their teknopods.