In the first book the main character is recruited (?) in a mission to save Earth by an alien AI with immense powers. He absconds with two robots, initially with no personalities, but radically enhanced by the AI. He steals a ship which is modified by the AI for extreme speed in order to intercept an asteroid targeting Earth, sent by an enemy which destroys entire civilizations. In a later book they come to a galactic mega city in space, and also descend into a sentient star to remove "dark matter" placed to destroy it. I started the series but lost the books.
I read the books at least 6 years ago. I lost the books during a move. I can't remember the cover art. I was into the third (or 4th?) book of the series. I'm not sure if that was the last one at the time. The last book ended with the main characters at the mega space station (a habitat with thousands of species). At that point he is surprised to find out his girlfriend from Earth may be there (since as far as he knows he is the only contact with the civilization). There is the main plot point of a an immensely powerful and secretive enemy seemingly bent on wiping out civilizations.