Hello! We see where you're coming from, however, when we refer to attorney x assistant we are referring to the non-attorney characters that accompany the defense attorneys both throughout the trials and investigations. These characters are usually much younger than the attorney they accompany (such as Phoenix and Maya/Pearl, Ryunosuke and Susato/Iris, Apollo and Trucy) and we want to prevent that gray line of 'Maya is 17 in one game but 18 in the next' when it comes to ships. Although Diego helped Mia in trial, he would not qualify as her assistant as he is her coworker who is also a defense attorney helping her with a trial. Similar to how Kristoph is not Apollo's assistant, that is his boss helping him during his very first trial.

We hope this clears up any confusion!