Samilin-Corrath-Gahar was an Andalite, and captain of the Ascalin. He was a traitor, voluntarily working with the Yeerks even though he wasn't a Controller (Ax considered and rejected the idea of him as a Controller on the grounds that it was impractical to assume that he would have been able to keep a Kandrona generator secret on a ship the size of the Ascalin).
When Samilin accidentally 'picked up' the Animorphs during a mission to Leera, the Animorphs having been pulled into the Z-space -displaced mass while morphing mosquitoes, he presented himself as just another Andalite captain until they arrived at the planet. During the subsequent attack, he crashed the ship and cut off his tactical officer's tail, intending to hand the Animorphs over to Visser Four, but he was crucially distracted when Cassie - who was hiding on his back as a flea- began to morph back to human, drawing Samilin's attention long enough for Ax to disarm him and his now-conscious T.O. to kill him with his discarded weapon.