Spike (Code of the Krillitanes)

Spike was a human child from London in the 21st century. He first met the Tenth Doctor when he was playing football. He amazed the Doctor by coming across as being very intelligent, which his mum credited to the Brainy Crisps. After defeating the Krillitanes, the Doctor visited Spike again and found his intelligence had gone back to normal. (QR: Code of the Krillitanes)