It hurt seeing Tetsu not telling Naoya and Yuuma about being with Mogumo, how it made ze sad by not introducing after confessing to each other so eagerly earlier, but he had to set the mood for dropping a bomb like that, even told about Satori being his sister and where she works.
I'm glad Tetsu has such accepting friends, it's a shame that's very much fictional, but really heart warming how Naoya took the initiative and by reading the mood a honest talk Tetsu set ground for was more importantly about Mogumo, even made ze came out of being just a background character in their talk by taking interest in zir hobby... gamers be gamers lol