Shironaga

Canon
Angie and Tsumugi rarely interact throughout the game. However, Tsumugi was one of the four students to join the student council despite everyone saying Angie was manipulative and it would be better not to join. Tsumugi insisted that Angie meant well and seemed to believe in Angie's faith. The scene in which Angie encourages Tsumugi to join is one of the things that fueled the ship. With Angie hugging her tight and insiting that Atua will always be there for her and Atua can be or look like whatever she desires.

There's also the secret event in chapter three when Tsumugi, Angie, Tenko and Himiko are changing to go swimming and in that scene Angie asks if she can touch Tsumugi's Breasts since they look big. Tsumugi simply covers up and starts stuttering and blushing while asking why Angie would like to do such a thing. Tsumugi states that she hates it when people comment on her body or say sexual things about her, however she seemed more embarrassed then she did angry when Angie was saying things in the locker room.

Tsumugi was also one of the few characters to cry over Angie's death. Angie wasn't well liked by many yet Tsumugi wanted justice for her and seemed more distraught then she did with most other characters. Tsumugi that said she'll avenge the killer at all costs. However with the reveal of Tsumugi being the mastermind it is possible that she was faking it. Despite this, Shuichi mentioned how she seemed genuinely sad at Angie and Gonta's deaths to which she ignored his comment and continued on with her speech about how they were all fictional.

Fanon
The ship sailed due to the interactions between the two and how Tsumugi was one of the few characters who genuinely liked Angie. Lots of people theorize that Tsumugi actually cared about her. They're also shipped due to their interactions in the bonus locker room scene.

The dynamic of a cheery energetic artistic girl and a shy awkward artistic girl is a trope that a lot of ShiroNaga shippers like to emphasize on. In fanfictions of a Non-Despair AU, lots of people like to portray as Angie being the one to start flirting and make the first move.

A lot of people also like to portray them together because both of their talents involve the arts. With Tsumugi being the Ultimate Cosplayer and Angie being the Ultimate Artist. Lots of fanarts between the two portray them sharing their talents together. Although most people prefer the ship in an AU where the killing game never happened or Tsumugi wasn't the mastermind.