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Both Tsu and Sirius have animal-like features from their quirks, Tsuyu's frog abilities and Sirius' strong hearing. Along with them giving the two a connation to water. Sirius is sidekick hero, while Tsu is training to be a pro hero at UA. Like Tsu, Sirius is friendly and this allowed the pro hero to bond with Tsu.
Tsu and Sirius met each other in Tsu's internship with Sea Rescue Hero: Selkie, and both of them have become good friends since then. We first see them having a little chat with some soda and they talk about how boring the internship was. They talked about cleaning, patrolling, training, and even laundering everyday. They said that it was tiring at first, but as Selkie's sidekick, Sirius learned a lot of stuff from Selkie.
Later on, they get a mission about finding a stowaway, Tsu gets worried that they might not find the stowaway, but Sirius cheers her up, telling her not to worry. When they find the stowaways, Sirius gets caught and Tsu tries her best to keep Sirius safe. As Selkie comes to save them, Sirius is worried about Tsu and asks if she is okay and Sirius does the same. Eventually, they were saved by the captain. When looking for a Hero Internship, Tsu wanted to pick Selkie's internship with Sirius, but sadly, Selkie didn't accept Hero Internships.
After the internship, Tsu goes back to U.A. High School. As the students were talking about their internships, Tsu explains the patrolling and training that she had to do and that she caught the stowaways in her internship.
SiriuTsu isn’t very popular due to ships like TsuChako, TokoTsuyu, and MidoTsuyu. Many people only consider Sirius and Tsuyu friends that met in their internships. While there are those who see Sirius as a big sister to Tsuyu, due to the sidekick being older than the young U.A. student. Those who do portray them as a family, sometimes have Selkie as the father of their ocean hero family while Sirius is either the big sister or the young mother to Tsuyu.
On AO3, the ship only has three fanfics. Along with Tumblr and Deviantart having a few fanart images of them together, as friends.