Loki

Loki is a rogue Asgard scientist. While the majority of the Asgard were searching for a new home after their previous home was destroyed by the Replicators, Loki came to Earth in order to do genetic research on Colonel Jack O'Neill. In order to do this Loki cloned Col. O'Neill so he could study the original for a prolonged period of time without arousing suspicion of the SGC. Due to a marker placed in Col. O'Neill's DNA, the clone ended up being in his teens rather than fully mature; this made the SGC suspect foul play, and they soon found that Loki had been doing non-sanctioned experiments on humans for some time. SG-1 captured Loki and informed Thor of his actions. Thor took Loki into his custody.