During the perfect ending, when the President says "Even though we tried to destroy him, he still saved us." after the comet is destroyed. And many responsible for the original attack on the station join in with many a My God, What Have I Done? lines to go around.
In the pure hero ending, Shadow stands triumphantly alongside his comrade Sonic with the seven Chaos Emeralds before recalling that faithful promise he made to Maria in protecting the Earth, one he is fully intent on keeping. Especially since he is on pretty good terms with Sonic and his friends now.
The slightly hero- Dark ending. Unlike the other endings where he ends up killing Eggman, he tells him to leave the sacred ark and never return, swearing to guard the place that was his home as long as he lives.
In the semi-hero ending, Shadow defeats Black Doom to make up for being the cause of incident on the Ark. This is in spite of being revealed to be a Black Arm like Doom. After realizing his existence was meant to cause pain and suffering, Shadow gets depressed. Vector tries to cheer him up in spite of hearing the Awful Truth about him.
Shadow's dialogue with Maria in Lost Impact and The Doom.
Some of the post-game messages are this. When you replay the game after completing it, you would hear your ally or an important NPC communicate with you (the replaying of the game is stated to be one massive simulation.)
One important one is how the GUN commander will actually inform that he has become a grandfather. The fact he is telling Shadow this shows the development the man has gone through in his relationship to the ultimate lifeform.
This one is quite subtle, but it's still there. G.U.N. are asked to hunt Shadow down, right? Well, Rouge is also a G.U.N. agent, quite possibly high up in the ranks. And yet, in the hero missions, Shadow is still willing to help her out, and Rouge appears to not tell G.U.N. what is going on. It's as if she's trying to protect Shadow, even at the possible risk of being fired. It shows how far the two have come since the events of Sonic Adventure 2 and Sonic Heroes. True Companions indeed.