6. Both Have Sibling Problems
Now, it goes without saying that the problems Thor and Buffy have had with regards to their siblings are very, very different - but the fact is they both have had problems involving their siblings that have had potentially world-ending ramifications. In addition to his fight with Loki in the first Thor movie, Thor had to face off against his adopted brother in the Avengers, when the mischievous sorcerer acquired an army with the intention of taking over the world and putting an end to humanity as we know it. In very different circumstances, Buffy had the problem of knowing that her sister - Dawn - was essentially an artificial being made of pure energy and was apparently the only means of opening and closing a portal that unleashed hell on Earth. Of course, both Thor and Buffy would, inevitably, overcome these problems in their own ways (Buffy by sacrificing herself, nonetheless), but they both initially had some seriously massive dilemmas and conflicts regarding their siblings.