While some same-gender relationships in anime have aged poorly, the relationship between Rat and Shion remains an example of a relationship done right even in the midst of their struggle to survive. Through carefully building a romance that centers on mutual respect, support and understanding, all within the framework of a dystopian society and the fight against it, No. 6 proves it is possible to have LGBTQ representation in any sort of story, and continues to be one of the best examples of m/m romance in anime today.
A number of factors lead to why Rat and Shion are so important, the first being how their relationship is developed and portrayed. Their relationship grows gradually and without any fuss over their gender; their compatibility is the most important factor. Rat or Shion could have been replaced by a female character and the plot would have remained just the same, and their sexuality is not the focus of the anime because there is a whole different story to tell.
Read on… (contains spoilers)