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 a 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. (NSA: Code of the Krillitanes)