So I totally ship these two after seeing them together in the movie and ive been wanting to draw them for awhile.
 
Headcannon ahead:
 
Following the events of the attack on Canterlot, Tempest is quickly arrested and put on trial. To Tempest’s surprise, Twilight took her side and claimed that Tempest could become a better pony, that she was only misguided and lonely. After some consideration, the princesses decide that they can forgive her since she showed true remorse for her actions and promised to do she could to fix the mistakes that she made, but under some conditions. She was allowed to remain free, but must stay with princess Twilight under surveillance and must participate in friendship lessons and public service. Tempest accepts this punishment and accompanies Twilight back to the castle where she is given quarters to stay in while she carries out her sentence.
 
Tempest is shy at first, but slowly opens up to the main six. Helping Applejack with the harvest, collecting gems with rarity, and participating in some animal tea parties with Fluttershy. But to everypony’s surprise, she hits it off really well with Pinkie Pie when it comes to baking a cake for a party. It turns out that Tempest’s special talent is baking and she lives up to her real name: Fizzlepop Berrytwist. She has just hidden it for so long after leaving her little village and being accepted under the Storm king. She didn’t want to come off as some fluffy little pony that liked to make sweet little cakes to the Storm King and his brutes. So she gave herself a new name: Tempest Shadow and made sure that she was feared and respected.
 
It also turns out that Tempest is an avid reader and has read a lot of books that twilight has when it comes to legends and history. They were a big part of the culture in her village and she just didn’t stop reading about the legends in the story books like Clover the Clever and the ancient StarSwirl the Bearded. Twilight is quick to jump on this hobby and ropes her into discussions about legends of the past and provides her with several books to read.
 
Tempest quickly finds herself spending more time with the princess than anyone else in Ponyvillie and recognizes that she’s catching feelings for the Princess. She feels conflicted though and the guilt of her past actions weigh on her and wonders if Twilight can forgive her enough to like her back.
 
It’s nearing the end of Tempest’s time with Twilight and she decides to take her out to a little dinner as a way to say “thank you.” She was planning to keep her feels to herself, but since Twilight is the princess of Friendship she can see clearly through Tempest’s facade and asks her what is going on. Tempest sighs and admits that she has had feelings for the Princess and that she didn’t think that Twilight would like her back.
 
To Tempest’s surprise, Twilight gives her a big hug and sheepishly admits that she was feeling the same and that she didn’t want to put the mare in an awkward position since they’re staying in the same castle under the Princess’s commands.
 
So Tempest officially moves into the castle and is given the role as Twilight’s royal guard (and marefriend). To Tempest’s surprise, this news is well received and she finally feels truly at home in Ponyvillie and that she can actually make a new life for herself. The ponies accept her and she has a side job working at SugarCubeCorner as a baker.