Huh, I figured Diogee-2 would've become a angry guard dog due to the trauma of losing Milo and being forced unground.
I think that would've been a little uncharacteristic of Diogee. Hes not a very angry dog. He is a very protective one, and hes got more love in his heart than rage. Sure, he hates and distrusts the Normbots and the Emperor, but hes also intelligent and at the end of the day, ultimately a Very Good, sweet and cheerful Boy.
I don't think he minds being underground so much. Thats where Sara is. And im sure he has little issue wandering around topside in their spare time whenever she doesn't need him, she doesn't keep him on a leash. I dont think the Normbots have ever classified Diogee as a threat. He continues to make many friends up there and helps as many people as he can. He really is a lot like Milo himself.
Just like Sara, Melissa and later Zack, Diogee continues to hope that the Murphy's will return, and they can be a family again. He misses Milo a whole lot. Sometimes he sits close enough to the steps of the Mansion just to watch the balcony and the rotating guards just to hope to get a peek of Bridgette, or Martin, or Milo himself.
Hope keeps him sweet. Hope keeps him fighting and protecting. So long as he has Sara, he will continue to fight. This is the dog he knows the Murphy's need.