Ohmygosh they have the same last name. I just noticed that
They're cousins!
Pearl barely ever assists you, so she really shouldn't even be on the list. She's super adorable, though. I should really continue playing Dual Destinies.
Anyway, Maya. I like all of them except maybe Kay. But Maya tops my list. I'm sad that Capcom never brought her back in the main series after 3, other than that brief mention in DD.
Xenogears is the tragic story of how your whole life can take a crappy turn, just because you happened to see a lady in a wedding dress before her wedding.
This boy is crackin' up, this boy has broken down
This boy is crackin' up, this boy has broke down
You're also forgetting that Maggey does assist you during the first section of the second game's first case. Though that doesn't really count as a proper assistant.
You've got to go for Maya. Trucy is a decent assistant, but she helps you too much, in my opinion. Maya's outbursts in court are just hilarious, particularly in the crossover game.
Kay was pretty good as well. Not so much during the actual questioning (seeing how she spent half the time being a suspect) but was vital during clue gathering.