So I think that AJ establishes a huge contradiction in Phoenix's character: he has undying faith in others but is extremely cynical. This actually adds a lot of depth to his character... If you take time to think about it.
Take lil baby Phoenix. He walks right up to Redd White, Dee Vasquez, and Manfred von Karma like "look at my conclusive evidence!" and gets arrested, mobbed, and tased, respectively. He loses his badge because he takes evidence from a random stranger.
Then take hobo. He has very little faith in Apollo ("I knew you could do it." "Really?!" "No. I just thought it would make a better entrance than, say, 'hiya'."). DD Phoenix said that he found Apollo to be more loud than reliable at the time. Feen proactively forged evidence "just in case". He literally trusted nobody except Trucy.
DD shows a far more cautious Phoenix. He still has tons of faith in others, but as soon as he got to the scene of the crime in turnabout academy, he started scoping it out under the assumption that there would be forged evidence.
Phoenix gets a lot smarter after losing his badge. He's always coasted on victory- his previous errors were just part of the case. But then he lost everything, and he learned that bad things happen to good people, and you have to be proactive in order to prevent that. He never lost his faith-- unlike Apollo, since Phoenix was the one to defend her to the very end. Phoenix believes in the good he senses in others, but understands that good people do bad things-- like Trucy and Vera, the girls who made and delivered that forged evidence. I think he believed that Apollo was a good person, but he obviously didn't believe in his abilities.